I never kept discus but I do know there are tons of articles and even pictures on the web of otos sucking on discus. They are attracted to the slime coat. Whether they actually cause problems for the discus seems debatable. Some people remove otos from the tank once they have acquired a taste for fish slime. Other say it only happens on occasion and causes little to no problems. And others say it can be avoided all together if you keep the tank well planted and feed the otos tons of veggies. Discus are expensive fish though. If I ever did keep them I wouldn't want otos attacking them
You could always go with an algae scraper and elbow grease.