Help Please! Someone is using my farmer in stardew valley and I don't know what to do!

[ChaoticG]

Someone is using my stardew valley character in my friends game to join and creep out me and my friends. I have 2fa enabled, my passwords are changed, no one else has access to my steam account! I don't know how it happened. I have no viruses on my computer. It happens on an on and off basis. The first time it happened someone joined my friends game and said my name and a friends then when my internet got back on it left. This has happened 2 times more now. Once A few months ago, and now again today. I need to know what to do! Please Help Me! (I attached a file of it happening)(My character is the one in the black shirt)

 

[winterten]

if you are not the host of the game then that might be why. Anyone invited to a multiplayer game can pick from any of the farmhands.