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I've had my set up of Opae Ula for about 4 months now in a 1.5L jar. There's only 8 of them in there and they've been living pretty happily so far. But recently I noticed there's a weird layer of sediment caked all over the ground and plastic plant I have in the set up, as well as some black mould looking thing near the bottom of the jar where the Volcano gravel rock meets the water. I don't think it's algae, because it's more grey or brown than green though there is a slight tinged of greeness, and they're covering the walls as well and are really difficult to scrub out (I'm only scraping the front glass and left the rest as is). I feed them a teeny tiny 1/8 of an algae wafer ever month or so since theres only 8 of them and i hoped they would clean the rest of the dirt or whatever it is but they seem to leave it alone. Does anyone know what is the cause of it or if its a concern? If its not a concern I don't mind leaving it as is since the shrimps look happy enough but I havent gotten any green algae growth yet in 4 months and the hair algae that used to be that came eith the shrimp in there somehow browned and died out.

Ps: I know the marimo ball is not best suited for brackish water but it was from another shrimp set up that ran out of space and I just needed to put it somewhere and it's still doing well and my shrimp love to hang out on it.