Probably a sponge. Now, was it always there? If it has been growing at a fast rate, then it is likely not a sponge. Sponges (like all animals) are heterotrophic, so they can't utilize nutrients and sunlight for food. Instead, they use their collar cells to draw in food through their pores and amoebocytes to distribute the food. They feed on plankton in the water column. Alga and photosynthetic bacterium utilize nutrients and sunlight directly from the water column. Now, the abundance of light and nutrients are FAR more prominent than phytoplankton in aquariums. Therefore, the rate of growth experienced by sponges would be far less than the rate of growth for algae and photosynthetic bacteria.
In short, if it grows and spreads quickly, it is a problem. If it grows and spreads slowly, it isn't going to be a problem. Since it is obviously not a cnidarian, it can't use chemical warfare or nemotocysts to harm organisms in the tank. I can't see a way that it could produce toxic substances either. Even algae isn't harmful, it is just unsightly. This stuff imo isn't ugly...but I certainly wouldn't want it to overtake my tank.
Looks like a sponge, but I guess it could possibly be (oh crap...the word is escaping me). Anyway, the other thing I think it might be isn't harmful either...
Anyway, sorry to rant. Answer my question (growth rate and if it was there when you started) and don't worry about it
