Curious ??

[Robert1234]

Unsure what this dark spot is, sponge or plant ? there are a couple hidden away.

 

[Tahoe61]

Looks like a nusiance algae Lobophora ( not my image). If you could take a piece and put it on the sand or against a light back round it might help with ID.

 

[ahiggins]

and is it bright blood red?
If so, I have spots here and there on my live rock of it. From what I remember its just a sponge