Why is there water stain on my monitor?

This is a problem with the monitor. In this case, it is better to send it for repair.

Generally speaking, there are ripples flashing or distorted on the screen of the display. It is caused by the poor performance of the power filter circuit inside the display.

Specifically, your fault is caused by the power ripple interference caused by the failure of the "power filter capacitor" or "one tube in the bridge rectifier" in the LCD or the desoldering of parts with excessive temperature rise. Normal operation can be resumed after replacing corresponding parts or repairing welding.