Algae or something else?

[Mikosreef]

Can someone identify what this is.
During mid day it turns dark (left side) and then fades away/disappears in the afternoon and at night when the lights are out.
The pictures are about 6 hours apart.

 

[Timfish]

I'm going to guess some kind of sponge. Part of the reason I for my guess is it looks like it's expanded more in the left picture than the right and some are able to inflate and deflate pretty quickly. I would also guess you could carefully lift out the entire blob of "sponge" and sand in one mass. I have black sponges that are not invasive but there is one species (genus) that is very invasive. If it was in my tank I'd keep an eye on it and if it seems to be spreading or growing quickly carefully remove it.