It looks like turf algae. I dealt with the something very similar when I started my first reef tank with 100% MarcoRock. It battled it for 6+ months and it eventually grew over almost the entirety of my rockscape despite having a very large CUC including urchins, dozens of snails (probably 50+), pithos crabs, emerald crabs. I scrubbed down the rocks at least a dozen times including 4+ times pulling out all the rocks and doing a deep clean outside the tank.
I hope you have better luck. The turf algae I had was super dark green may be be different. I ultimately did a tank reset where I replaced all my rock with 100% ocean live rock. No major issues with nuisance algae in the 9 months following reset.