Two people that I don't know joined my world.

I left the game but when I came back there were some chat messages in grey and the chat color was default white. I got these messages from two people that weren't on my friends list with the usernames Vibe P1 and VisedPopcorn790 though I never actually saw them in game and i'm pretty sure that they got on my game when I was offline and it never said that they joined or left the game.