Looks like some sort of algae. Reduce the amount of light your tank is getting and put the live plants back in -- they use the same nutrients that algae does and will help soak up that "food" instead of the algae. Don't dose any fertilizer for now except Excell (CO2 supplement) if you have it. You can clean the algae but your best bet is to kill it by starving it.
The tank doesnt get a lot of sunlight but maybe it is. I'm setting up a ten gallon for him so that he will have more space, but i'm not sure if i want to put the same plants in there. Do you think they would spread? And i dont use any fertilizers.