Open up the controller and see what's going on inside. I can't tell From The pic if it's burning or if it's just an external mark from Something else? If something is wrong you will see blackened connections and burn marks inside. Then you definitely know something is wrong and stop using it immediately!
The screw in the top left corner has a ring of suspicion about it.Is it longer than standard perhaps?
Thanks for the replies guys. I've had a look inside and can't see any evidence of heat. The screw didn't screw back in properly. I've had the controller for 7 years with no issues. Never even had to change the fuse. Not done any pressure washing for a while so will have another look inside and hopefully get a photo.