Sponge ID?

Setup new tank about a month ago with live rock. This strange substance (not even sure if it is sponge or not) was on a good portion of the biggest rock I received (Gulf rock).

It is slimy and it is like a coating. Brown in color. Does not come off the rock very easily.

It looks like it is starting to spread to the rock above as you can see in the picture.

What is it? Is it bad? How do I remove?

Originally thought it was algae, but was told it might be a sponge.

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