Looks like classic hair algae. Bad news is you have a lot of it. The good news is you can beat it (unlike some other varieties of algae that are much harder if not impossible to eradicate).
How long has the tank been setup?
Start large (50% if you can swing it), weekly water changes with known good quality RODI water (no tap water) as a base. When you do the water change, use a smaller diameter hose (1/2" ish) to suck up as much of the algae as you can. If it has a decent hold, it most likely will not come off the rock by just siphoning it. You will have to suck some of the algae into the hose, trap it with your finger and pull the hose away from the rock essentially pulling the algae off the rock. If you can get the rock out easily, it is better to take the rock out of the tank and scrub or even power wash the algae off the rock.
Second part of the attack is to reduce feeding. How much are you feeding, what types of animals and how many are you feeding?
The third part of your attack should be adding some clean-up crew that likes hair algae. Mexican turbo snails, emerald crabs, and/or a tang in the bristle tooth family. I have a Kole Tang that keeps my tank very clean and loves hair algae.
Lastly, if you have Nitrate and Phosphate test kits, it would be helpful to know these levels.
alt="" />
alt="" />
alt="" />
