ID, weird white stuff growing in tank

[rnr_reefing]

I noticed this on the backside of my tank. Didn't look like a sponge to me, and is growing in weird patterns. Almost kind of looks like it has a smooth texture. Is this a bacteria or fungus? Should I leave it alone or remove it?

 

[saltyfilmfolks]

Those are some really really cool sponges.