What type of algae is this?

I have watched this grow on one of my rocks for several months. I do not have it anywhere else to a substantial degree in my tank except on this one rock. There are small amounts on other rocks but nothing like this one has. My corals are all growing well, including my SPS. By parameters are good, i.e. nitrate, phosphate near 0. Calcium, alk., pH are in range as well. My tank is 3 1/2 years old and otherwise doing well. I cannot scrape it off either. I have tried to scrub it with a toothbrush and it will not come off. I have also tried treating the tank with Algaefix and Cyano Stain Remover without any improvement. Any idea what this is? I have even thought about taking the rock out entirely and replacing it. I would rather keep it because I like it's character. Some of my fish like to hang out in the holes and tunnels that it has. Thanks.

Leonard

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