Looks like cyano to me. If you put a siphon on it and it comes right out its cyano. Excess nutrients is usually the cause. What size is the tank and his long has it been running, what's your water params, tank maint. and feeding schedule like?
Using RO water, ammo 0, nitrite 0, nitrate 20ppm, ph 8.2 , temp 76, 3 small damsels, 2 small clowns, 1 banded shrimp, 5 hermit crabs, feed 1/2 a cube of brine shrimp in the evening once a day, do 25% water change either once a week to week and 1/2. Equipment is in signature and protein skimmer remora-s. Live rock and base rock in tank.
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Been running since before Thanksgiving, and yes I can siphon up
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