6g office nano, 20g aio cube, 2 x 40g breeder community and 75g
It looks like the beginning stages of black brush algae. Commonly referred to as BBA. There is an imbalance in your tank and it may not be one thing. There are spot treatments with peroxide you can look into, but it's better to fix the tank in the long run. Search the site to fine cures and treatments.