Possible Hacker Joined Server (Or just coincidental server error)

One of my players was on my server earlier and said that when a player joined he got random text on his screen. I am concerned if this is a possible hacker or not.

 

Not a hacker.

 

Alright cool. I wasn't entirely sure because I haven't seen those before and it happened as soon as he joined. I didn't think he would be because that text isn't something that would normally happen if someone tried to hack it, but you can never be too sure.

 

Error with rust. Happens now and then, it's a problem with the actual game