Any clue what this is?

[vegasgungirl]

So I was given these dendros to try and save and it came with this orange patch of something on the side. That orange thing grows and shrinks as the fish swim by. I was told it probably isn't sponge because sponge doesn't expand or shrink quickly. Any one take a guess?

Just keep swimming

 

[Tahoe61]

Tunicate perhaps.

 

[tampasnooker]

+1 Tunicate/sea squirt