Yeah, I think it does look more like rhizoclonium. It's definitely more green and doesn't have any blue. If I pull it out it's slimy and stringy.
Could it be lights too bright and inconsistent co2? It's a new tank without any fish, dosing EI and I'm pushing a lot of co2. I see it mostly around the HC and near the co2 diffuser.
I've got a cheap fluval co2 though which is hard to keep consistent. I've got a new regulator and tank coming in next week. Do I need to get aggressive with this stuff, or slow and consistent with better co2 the way to go?
Any or all of that could cause it. Get the CO2 straightened out as soon as possible. Not sure how much light you have, so that may or may not need adjusting a little.
Its common for new tanks to go through a few algae phases during the first month or three. Diatoms and green dust algae especially. If conditions are clean, and favorable to the plants, they usually run their course and disappear on their own.
This stuff? I really dont know. Maybe somebody else has direct experience with it.
Id just manually remove as much as you can, get the co2 right, do an extra water change mid week for a month or so, And see what happens.
If it gets worse instead of better then it might be time for a more aggressive treatment. But that usually works for most algaes.