Assuming it is black ich, there are two species of turbellarian flatworms which are known to parasitize marine fish: Paravortex sp. and Piscinquilinus subcutaneus. Paravortex are smaller and appear as grains of black spots rather than lumps. Piscinquilinus are relatively large and lie deep within the dermis(skin). As a result, a pigmented dermis may show once killed resulting in a black spot regardless of a living parasite within the skin.
there are a few methods you can utilize to battle this. Although PraziPro may help, this is not the type of worm or a fluke that you are battling.
You will need to treat with an anthelmintic type of treatment. begin with FW dip – 5 minute freshwater dip may only provide temporary relief, and not result in complete eradication and assure temp is same as display tank. As long as you are utilizing a FW dip, you can also do hyposalinity at 1.009 SG for at least 10 days. This kills both the infective and free living stages of the life cycle. My favorite is Formalin as a 45 to 60 minute bath treatment, followed by transfer into a sterile QT afterwards.
Reinfection will occur no matter how effectively the fish have been treated if these parasitic turbellarian worms are not eradicated from the main aquarium and for this reason, I would consider going fallow 45 days of no fish (hosts) in the tank. I personally would go 60 days.
Pictures will be helpful