Originally I thought it was a simple algae. It might be slimy dino's, or algae, or cyano. All can be fixed with a little local water motion. If it were vermedits, we would see the little snails around that area.
Edit: Here is the other thing I would do. Re-assess the flow strategy of the tank. That means maybe adjust a powerhead to blow in a different direction to get more flow locally, or maybe aim one long ways and turn it up, and let the return flow do more for you there. Flow is one of the hardest things for reefers to simulate in their heads. I feel like I have an ok time with it myself (can get it right 6 out of 10 times) and I got my masters in fluid dynamics.
Play with your pumps and see what that does for you!!!