Any ideas what this is?

[The Grizz]

+1 on a sponge, I have a few of them in my overflow's and sump.

[cwatkins]

Give em enough time and your overflow will be filled with them.

Mine is, but I do try to scrape them out of the sump if I ever see any. And also pick them out of the DT if they start growing.

[Siderjan]

Thanks everyone. I should remove them then? I have 4, and they are the same size as they were when I go them a month ago.

[Bloodasp]

If they are not in plague proportions I wouldn't bother removing them as they don't do anything harmful to the tank and might even benefit it.