Please Help Identify

[sfoley23]

Good Evening,
Can anyone help identify what this is? Originally this thing we never saw move, so we did not think it was a creature. However when I woke up this morning it was not in its original spot and it was in between these rocks like in the picture. Later on today I noticed it had moved and is nowhere to be found. Any help is appreciated we took over this tank from someone else and are new to this.
Thanks

 

[sfoley23]

The top picture is where we discovered it when it moved today and the bottom pic is where it normally used to be underneath one of the rocks

 

[cristata.reef]

It's a type of sponge. How it moved, I have absolutely no idea