We're here to help and I hate telling people they cant or shouldn't add more fish, but in your case unfortunately its one of those situations, i think you're over stocked and under filtered. Try doing more/larger water changes, high nitrates from lack of fresh water can cause algae growth to spiral. The fish themselves look plump and well fed, try cutting back alittle when feeding, this will also lower nitrates in the long run which could also help with algae growth.
I like to say something positive and i like the fact you have a bare bottom for easy cleaning, that's the way to go in a tank like that.