Help me with white substance

[Clementskimberly3]

Can yall help me identify the white substance? (Not the aptasia, my shrimp will get that) This stuff lives through water changes and scrubbing. I have it on two rocks. The third rock I took out.

 

[EnterName]

It's really hard to see in the image.
I could imagine it's a sponge. There are many white sponges (for example post #82 on page 5 in my microscopy thread). Especially if you can see tubes or holes in the structure it points towards sponge. Not all sponges are harmless, but if it doesn't grow aggressively and doesn't overgrow corals it won't be an issue. Not all sponges are soft, many are encrusting which means you won't manage to scrub them off easily.