Hi Peter, helped hobbyists through this and it works. So far you’ve done two things: changed filter socks and manually removed the algae. So this could be more a phosphate issue than a nitrate issue so cleaning your filter socks (which is important) will do more to keep nitrates low. Phosphates feed algae. Test them. Start using gfo (as directed) or Phosgaurd (as directed). Don’t stop until the problem is gone.
Next Lighting: what is your lighting schedule? How old are your bulbs? Is your tank in direct or near natural sunlight? If you can cut back on lighting schedule by 30% (from 12 hours to 8 for example). Do this as well until the problem is gone.
Purchase CUC! An aggressive cleaning up crew of 20 or 30 Turbo Snails more depending on the size of your tank.
These are ongoing things that continue while you’re sleeping or at work.
Blow off your rock work with a turkey baster. Agitate the algae. Keep manually picking it off. Then toothbrush it into oblivion! Seriously, as aggressive as this stuff is, you need to be more. Good luck, Happy New Year!