Just spotted this tonight. What is it?

[islandboy73]

It's evening and the tank lights are off. I was walking by and saw the sand-sifting starfish was making his way across the sand so I grabbed the flashlight to take a look. I was looking at the rocks and noticed this for the first time at the base. What is that?

 

[ndrwater]

Sponge? With that color and base of the rock, thats my 1st guess

 

[PR_nano]

Sponge