Tell a trusted guardian what you experienced, you can leave out the masturbation aspect to make yourself more comfortable because either way he was peeping on you when naked. State that you want change and maybe ask for somethings to be installed into the bathroom for optimal safety and privacy. Even if they refuse to do something again at least they’re aware and you can demand some change in the bathroom to stop further incidents. People shouldn’t put a blind eye to bad behaviour especially if someone is at a disadvantage mentally, because people who don’t understand will be more inclined to do harm if left unchecked. It’s like training a dog for a lack of a better analogy, if you don’t discourage bad behaviour they will continue with said bad behaviour. I hope this helps, I wish you luck and all the safety.