[trenac]

Gab, welcome to APC... That looks like Slime algae to me. Called blue green algae (BGA) also know as Cyanobacteria. Most of the time it appears blue/green in color but also can be black, brown, purple, yellow and red in color. BGA is not a algae but are prokaryotes like bacteria.

You can rid your tank of it by using E-mycin (my preferred method) or a blackout, read this.

However to keep the this algae and other algaes from coming back you need to keep your tank balanced...Nutrients, C02 & lighting.

If you post your water parameters, tank specs, wattage etc., we can let you know if you have a balanced tank. The first thing I see is you need to up your lighting from 8 hours to 10 hours daily.

 

[trenac]

Gab... Your parameters look good and you are working on the N03/P04 issue; make sure to keep them at a 23 to 1 ration (exp:10ppm/N03 to 0.4ppm/P04).

The only thing I see is the daily lighting needs to be increased to 10 hours daily, plants need at least 10 hours up to 12 hours daily to photosynthesis correctly.

Then to initially get rid of the slime algae do a blackout or use e-mycin. Before starting the treatment clean as much off by hand as possible and do a 50% water change.

Also make sure to keep your C02 between 20-30ppm and do weekly water changes.