In that case, you have something called blue-green algae, which is cyanobacteria. It's a photosynthetic bacteria, and it's not good for your fish.
There are a couple of ways to get rid of it:
1. Tank blackout
- Remove as much of it as you can by hand
- Do a large water change (50%)
- Cover the tank with trash bags or blankets to block out the light
- Do not turn on lights or feed fish for 7 days (the fish will be fine!)
- After 7 days, remove the covers and do another large water change.
2. Erythromycin (antibiotic)
- Get some Erythromycin and dose as directed on the packet. You can get it at your local fish store, under the Maracyn brand, or other brand (which is usually cheaper).