Red slime. Can't remember whether that's cyanobacteria, diatoms, or dinos, but it _is_ a nuisance pest. Tends to show up in new tanks; I had some last spring after I added a frag tank to my system, kinda surprised me because frag tank is connected to my 2+ year old tank/sump.
Try turning off your lights for 2 or 3 days, that usually kicks it in the pants. It will eventually go away in a few weeks as rock and sand mature (grow beneficial bacteria, outcompete nuisance algae, etc), but if it bugs you right now turning off the lights will starve it.
Increasing flow to that area should also help. You could try pointing a powerhead at it.
I normally don't do anything about it, it will go away on its own and I've never seen it bother anything.