What is this stuff?

[redburn]

Hello everyone.
What is this and is it bad? It seems to be popping up in several spots in my tank. This is the largest and seems to grow rather quickly.
Thanks

 

[griss]

Looks like a sponge or sea squirt to me. Both are harmless and beneficial filter feeders.

 

[Guygettnby]

It's an alien! Better nuke it before it tries to take over your brain!

Seriously though, I agree that it looks like a sea squirt. As your tank matures, you will find tons of them hidden in your sump or filtration areas.