It looks more like sponge to me. I have a purple sponge growing on one of my rocks that looks pretty much identical to that. Even had my LFS
look at pics to be sure, as I was not sure what it was either when it first started growing.
How is the flow in that part of the tank? Any dead spots? Maybe play a little with your flow and dim the lights (if you can) and see what happens. I am definietely no expert, but it looks different than any pics of cyano
I have seen.