That's a STUCK BUTTON on your computer monitor.
Press and release all the front panel buttons one by one. You may find find one that FEELS sticky.
If it is sticky, you can take 70% to 90% alcohol and dip a toothbrush in it and scrub the sticky switch or switches, WITH the power cord disconnected.
That should work, but if it doesn't, then the Brightness up button needs replaced because it has failed.