They likely refer to Caliban as a monster due to the fact that in the play he was known as a half-man half-monster.In some traditions he is depicted as a wild man, or a deformed man, or a beast man, or sometimes a mix of fish and man, a dwarf or even a tortoise. Showing he is not man, but monster despite his acts.
I think it's important to tell a new friend who doesn't know about the disability that you or someone has because if they say or do something it can get defensive triggered or they might just not understand what you're coming from