New alien in my tank

[74maddox]

I've had my tank set up for about 9 months, no new rock or anything for at least 6 months. About a month or so ago, I found an "alien" on the underside of one of my rocks. It hasn't done anything- no growth, no movement. But, what IS it? I'm attaching 2 pictures in hopes that someone might know what it is. Thank you!

 

[flsalty]

That's a sponge.