Telnet disconnection errors are pretty normal, if its from RAT, RAT will reconnect. The connections in your screenshot, appear to be from multiple IPs. So it looks like someone was either doing a port scan, or otherwise attempting to telnet into your server. The red color just means the connection was closed (usually that they failed to sign in with the right password).
Ultimately, it isn't anything to worry about. But you can always change your telnet port (and reconfigure RAT to match it) if it bothers you.