Appears to be diatoms. As mentioned, diatoms can reappear in established tank especially if you added new rock or sand, changed your water source you typically or even changes to lighting. Diatoms feed mainly off of silicates but also consume dissolved organic compounds, phosphate and nitrates. Unfiltered tap water can contain silicates and is a good way to jump start a bloom if you use it to mix salt or to replace water that evaporated from the tank. The best way to prevent this from happening is to filter water through a RODI unit if youre not already, although you can still get a diatom bloom when using RODI if the cartridge that removes silicates expires. Take a look at your di resin as it may be in need of change.
Diatoms are typically harmless as you may know and can be beaten once their food source expires. Once you put the kibosh on the source, the outbreak should last a couple of weeks so just be patient and it will pass. For major outbreaks you may want to consider the three day blackout. To prevent their return, practice good aquarium husbandry by doing regular water changes, keep the substrate clean, don’t overfeed the fish, ensure your skimmer is running at an optimal level and rinse out filter socks and sponges on a regular basis.
Some cleaner crew to help control it are : Cerith snails, Nassarius snails, Nerite snails and Trochus snails and also Astraea snails are effective at removing diatoms.
