Sounds like a RAT ( Remote Admin Tool), as the name suggests he would have remote access to your computer. Probably from one of the random Exe you downloaded from the private servers. I have seens RATs have runescape specific tools where they are able to log your bank pin, the program is able to tell when you enter the bank pin screen and takes screenshots or screen records and sends back to the host.
I would just back up all your important stuff and do a factory reset of your computer to make sure it's gone, as it can go undetectable by anti-virus especially if the owner keeps updating it.