You have Blue-Green Algae. Needs to be treated with Erythromycin Phosphate, you can find the treatment at your LFS called Maracyn. That alone will not cure it though... You will need to scrub everything until you can't see it anymore, do daily water changes, and I highly recommend keeping the lights off completely for a minimum of 2 days.
Can I ask how often you did water changes?
Never ever ever treat BGA with antibiotics. This is a sure way to 'make' a antibiotic resistant strain and other super illnesses for your fish.
There are some important points of information we need before anyone can make an infomed decision on how to prevent the BGA from returning.
Specs plz:
- Tank volume:
- Light wattage:
- Number of Bulbs:
- Photo period (how long u leave your lighs on):
- Filtration:
- Stocking:
- Ammonia:
- Nitrates:
- Temperature:
- Fertilization regiment:
- Water change regiment:
From the looks of it, the main cause appears to be, too much light, too few plants and too little flow. I will personalize this depending on what you give as your parameters. One of these things wont work without the others.
1)You should get either another HOB or a power head (bga cannot grow in flow and are intolerant of O2),
2)raise your water level to minimize splashing (for O2 and CO2 stability)
3)keep your lights on for 6 hours after you see algae go away you can raise it to 8hours
4)get TONS of stem plants (8-10 bunches of Brazilian pennywort, cardamine, hornwort, watersprite, water wisteria etc). If when buying your plants you think you have enough, you don't and need more.
5)Keep up with weekly water changes to clear out that crap but don't let your nitrates fall below 10ppm; with TONS of fast growing plants you can eventually ease back on the water changes all the way to 4x a year(only with TONS of plants). Just monitor your nitrates and don't let them get above 40ppm
6)Dose ferts - your micro (iron, manganese, molybdenum, boron, sulfer, calcium, magnesium, zinc etc) and macro (nitrogen, phosphate, potassium, you can even lump iron, calcium, magnesium in macro too). Ferts fuel plant growth wich in turn will slow algae growth. Never ever stop ferts when u run into an algae problem, just reanalyze what your are lacking or have to much of and adjust accordingly.