Is this a mushroom?

Not sure what you have is this,but I would remove the rock and make sure it's erradicated.I had this algae in a nano and it quickly,in a few months time,covered most of the tank including powerhead and skimmer intake and pump.That tank is now fresh water,it's tough stuff.

 

To bad! it looked cool, hard to imagine that its an algae!

 

Yep that's what I thought at first,didn't care much as I pretty much neglected that tank most of the time.When powerhead and skimmer stopped working because of the stuff I physically removed as much as I could but 2-3 days later it was back just as strong.It's very tough,removing it was almost like scraping mussels from a rock at the beach.