WHAT IS THIS?

[RudyvandenBerg]

Was rummaging through the old aquarium and noticed this strange whatever covering the entire one side of a rock...

Its kinda firm but still squishy - like a toadstool for comparison. Any thoughts?

Some close-ups.

 

[dwest]

Looks like a sponge.

 

[KJ]

+1

 

[ScottR]

Reminds me of a carpet sea squirt

 

[KJ]

carpet sea squirts - - Image Search Results

images.search.yahoo.com

 

[Twisted1990s]

Pineapple sponge? Harmless; but a nuisance. You you need to manually remove.

 

[ScottR]

Pineapple sponge? Harmless; but a nuisance. You you need to manually remove.

Not pineapple sponges. These are colonial