@LaurenCP72, let me start with "Welcome to Reef2Reef".
As for what that is... it's a white thing with what looks like some yellow stuff at the top and it's attached to one of your rocks. OK, sorry for the teasing... I couldn't resist.
So is it hard or soft? How long has it been in your tank? How long have you had a this tank set up and running? How did you come by this 'thing'? Does it ever look different, as in dose the top ever 'open up' or change during the day?
I wouldn't be too concerned about it... yet. But the answers to the questions would help out a lot in trying to figure out that it is.
It could be a sponge, but but I think that's unlikely. It could be a soft coral, but I suspect the top should polyp out at some point. It could be a stony coral, but it too should polyp out at some point. That's why the questions above.
Looks like a type of sponge. Similar looking ones pop up in my tank but I think the angel eats them. If thats what it is, harmless and a sign of a healthy tank.