Weird growth on rock

[Juliuszkaras]

This started growing/appeared on one of my live rock Abt 2 months ago and I've been trying to figure out what it is since. Scraped some of it off but it grew right back. Any thoughts on what it is?

 

[cloak]

It looks like some type of sponge. I would just leave it alone for the time being. If you ever want to remove it though you'll probably have to remove the rock and use a wire brush.