You mean like this? They're sucking on the slime coat. It is potentially a bad thing, but my Ottos don't do it regularly. If they do do it regularly, then it would be a good idea to take the offending one out. Maybe they do it because they are hungry, I don't know. Feed them more food and veggies. Unless you mean they're stealing food from their mouths.
I had a pleco that started doing that. I removed him immediately. If I were you I would remove the offending ottos. The fishes slime coat is its first line of defense, and having it damaged could cause problems.
I'd also suggest removing the offending otos from the discus tank. Not only will their actions remove the protective slime coat, the constant torment will stress the discus and further weaken their immune system.