All I know is that people called him a pedo for making games for kids and interacting with them by talking with them on Scratch via the comments.
I've found something else that Mossy posted on his YouTube channel about why people call him a pedo. (I only briefly skimmed over it, so some information might be incorrect.) While I don't check his social media stuff, he says that on one of his social media profiles, where he posts art on accounts that have 18+ content on them, and people who are below the age of 18 are going to these accounts and looking at them because people tell them to do it to harass Mossy, which is also bad for minors who are being exposed to adult content.
Link to the post (if you can view stuff like this): Post from MooseShower - YouTube