first contact

I was playing Roblox today instead of Minecraft. Randomly someone named "Watcher" joined my game and friended me.
I switched games to the game I was playing with my best friend from high school earlier. But it seems like "Watcher" followed me anyway. I went back so I could try speed running the first world of the game this time.
I was in one of my private servers in a game called "Rocitizens" when "Watcher" joined the first time. I thought only friends could join my private servers but it seems that there is a flaw in there.
I will have to close all my private servers to prevent "Watcher" from joining again.