I think it may be the beginning of coralline but I could be wrong on that. It's certainly not a type to be concerned about and your inverts may consume it....
That's a normal part of the rock cycle. Different algaes will grow initially and periodically be taken over by another. Eventually the dominant algae will be a calcareous type called coralline algae. Can be shades of greens, purples, reds, and yellow. Your tank will go through much more dramatic changes so don't worry, what you have on the rock now is completely normal.