I have had similar looking things to the red piece you circled. I believe it is just a red type of algae similar to bubble algae. That said, I've never had problems with it "taking over" my tank or anything.
I'd let it go and see what it does. At the absolute worst, in a tank that size, it starts to spread out of control and you have to ditch a few pounds of rubble. If it were a bigger tank, I'd say take the rock out, chip the rock off underneath of it, and just get it out (better safe than sorry in most cases). But red algae can be pretty attractive and could be a neat thing to look at.
And if this advice comes back to bite you (you know where), I'll gladly supply the rubble to replace it.