I noticed a tiny snail attatched to my bigger Mystery snail. It doesn't seem to want to get off. Can it be harmfull?

It's usually nothing to worry about. Is it another apple snail (mystery snail's are usually apple snails) or another species entirely? Is it attached to the shell or body? If shell, then no problem. If a small snail sucked on the body of a larger snail for a long time, that could result in injury although I've never seen it happen.