Its a type of encrusting algae similar to coraline. Its desirable, and I would think a very good sign. These patches will grow will grow to a size of about 2.5in in diameter. (I've never seen it larger than that). One problem is, this stuff is growing and is a sign of your tank doing well, it can just as easily go away if your conditions get less than desirable, so you have to keep the tank in the same shape as it .