That is not Lymphocystis. I would say it looks like a parasitic copepod, but I can't rule out isopods. You will never see these on captive raised fish unless your tank is already infected with them. These clowns could be wild caught ocellaris - but if they are, then you may have to contend with Brooklynella as well at some point (sigh).
In 55+ years of keeping clownfish, I have never seen one infected with crustacean parasites, so I really don't know what to make of it. If you can get a clearer picture, I might be able to give you a better ID.
Here is an excerpt from my upcoming book about these:
Parasitic Crustaceans (Copepods, Isopods, and Sea Lice)
Cause
This disease problem is caused by a variety of very small crustaceans that can parasitize fish. Some are obligate parasites with specific host species, while others are generalists and can live separate from a fish and only return periodically in order to feed on the fish’s skin. The different species have a wide variety of body shapes, but because they are all arthropods, they always have jointed legs and segmented body parts.
Non-parasitic amphipods, copepods, and isopods are also common cryptic inhabitants of aquariums. Seen as little white specks moving about the aquarium glass, these creatures (often called “pods” by aquarists) are harmless but sometimes indicate that the primary inhabitants (the fish) are being overfed.
“Pod” characteristics
Amphipods tend to be flattened side to side and have two different types of legs. None of these are parasitic.
Copepods are flattened top to bottom, and non-parasitic ones often have large antennae. In parasitic species, the antennae are replaced with grasping hooks. So, if you see a large number of copepods, all with large antennae and not attached to a fish, they aren’t parasitic (though the parasitic species are free-swimming during the copepodite stage).
Isopods tend to have legs that are all the same (“iso” means “same,” and “pod” means “foot”). They are also either cylindrical or flattened top to bottom. The cirolanid isopods look a bit like pill bugs that you can find in your garden, but they are known parasites of fish.
Symptoms
Symptoms of copepod infestation can be a bit too general and vague for most aquarists to discern visually. The fish may flash (scratch), act nervous, or breathe heavily. In severe infestations, the fish’s skin will develop blood spots and the fins may become tattered.
In older aquariums that have a high bio-load (large number of fish), an aggressive type of copepod may take up residence. These creatures hide in the gravel and under rocks during the day and swim up into the water column at night to feed on the fish’s skin. Fish in aquariums with an infestation of this type of copepod will be seen hovering near the surface, as far away from the substrate as possible. They may also show other signs of stress, such as pale coloration, jerking movements, and rapid breathing.
The purple firefish, Nemateleotris magnifica has a copepod parasite specific to that species – Serpentisaccus magnificae. Described by George Blasiola in 1979, this parasite is seen on newly imported firefish. The body of the parasite is buried in the flank of the fish, behind the gills, either on one or both sides. When the female copepod develops its egg sacs, these emerge from the fish’s skin as tiny coiled strands. Treatment should not be attempted for this parasite, as the adults are buried too deeply into the body of the fish for physical removal to be done safely, and no known chemotherapeutic will eliminate internal crustacean parasites without also harming the host fish. This copepod is thought to have indirect development with a series of planktonic larval stages, so there is no real concern of the eggs hatching and then fully developing and causing reinfection of the host.
Diagnosis
Some crustacean parasites, including many of the isopods, are easily seen with the naked eye. In other instances, the female’s twin egg sacs can be seen emerging from under the fish’s skin. Branchiurans (Argulus) can sometimes be seen by observant aquarists, but as they are clear, their 10 mm bodies don’t stand out very well. Freshwater aquarists may find these on wild-caught freshwater stingrays.
In other cases, such as with Ergasilus, the parasites are too small to be seen directly, so you need to look for secondary symptoms as mentioned above. A diagnostic dip will aid in the identification of these pests. The dip also serves as a temporary treatment, as it rids the fish of most of the parasites attached to it at that moment.
Treatment
The most frequently suggested treatment for crustacean parasites is the application of an organophosphate pesticide, such as Trichlorfon (Dylox or Metrifonate). These products are dangerous for aquarium use; toxic to the aquarist, and causing sensitivity reactions in many species of fish. Other treatments need to be considered first.
One way to deal with these parasites in the aquarium is through a series of dips—formalin at 166 ppm for 45 minutes or fresh water for 5 to 7 minutes. Two difficulties are seen with this method, however. First, returning the fish directly to the infected tank allows for rapid reinfection. Second, these parasites have pinching mouthparts that make them difficult to dislodge during a dip treatment.
One method used by public aquariums is to give the entire aquarium a high-dose (166 ppm) bath with formalin. After about 30 minutes, the tank is very quickly drained to 20% full. Previously mixed seawater is then added to the tank to refill it. This process exposes the fish to about 45 minutes of formalin above 150 ppm and then leaves a residual formalin dose in the tank of 33 ppm, which is generally tolerated as a constant bath by most fish.
Obviously, this cannot be done in aquariums where invertebrates are present, and it does require a substantial amount of salt water. Furthermore, a second treatment is often required. For reef aquariums, removing the fish to a treatment tank and allowing the main aquarium to lie fallow for six weeks is the best treatment.
