What's in there.. LR? Cured LR? sand? How fine? Powerheads and sump return running to create flow?
It could be any number of things. Die off from the LR, sand in the water column (I bought much to fine a grain of sand and it seems there's always some floating in the column. Over time it will take on eight from the bacteria and settle or get filtered out. When it really was really bothering me I would hang an old HOB on and let it run overnight.)
Did you mix your salt in the tank?
Doubt it's a bacteria bloom but I suppose it could be.