I don't know if you've just got green water (free floating algae) or if there is algae on the walls of the tank/gravel, but if there's some on the decor/walls and you cut back your plec's food, he'll start cleaning things up for you 
BTW, anything your plec doesn't eat overnight should probably be removed. It's better to feed 1/2 a tab per day than 1 tab every two days if it's taking him two days to eat it. Any food sitting in the tank for a period of time will contribute nutrients that feed the algae.
Blacking out the tank for 3 days is supposed to really help with floating algae. cover it up completely for 3 days - allow no light in - and then do a big water change.
Phosphates (fish food is high in PO4) and nitrates will feed the green stuff - cut back on the light, do water changes and feed less for a while, and hopefully the dreadfull stuff will die off.
BTW, anything your plec doesn't eat overnight should probably be removed. It's better to feed 1/2 a tab per day than 1 tab every two days if it's taking him two days to eat it. Any food sitting in the tank for a period of time will contribute nutrients that feed the algae.
Blacking out the tank for 3 days is supposed to really help with floating algae. cover it up completely for 3 days - allow no light in - and then do a big water change.
Phosphates (fish food is high in PO4) and nitrates will feed the green stuff - cut back on the light, do water changes and feed less for a while, and hopefully the dreadfull stuff will die off.
