SAE's are hit and miss with discus as they can get a taste for the slime coating on the discus, the same goes for otos and most sucker mouth fish. saying this i have 2 SAE with my discus with no probs but ive only had my discus for a couple of weeks.....
its best to source the reason why the algae has begun rather than try to use fish, shrimps etc etc to keep it at bay though.