If its coralline algae it's a good thing to have. All that coralline algae basically is, is a colored type of algae that grows on live rock, powerheads, sides of the tank, etc... Can be in an assortment of colors. A very good type of algae to have. If that's wat it is, and if it's spreading, it's a sign of a healthy tank.
But it does look like you have a couple red mushrooms growing on the rock as well. Kind of hard to see on my phone tho. If they are mushrooms, they're a good thing to have. It's a type of coral that people pay to get from stores to add to their tanks, and if that's wat it is, then you got lucky with those hitch hikers!