Fluidmaster toilet fill valve not shutting off water
You need to turn off the water supply at the stop valve, gently prise open the Fluidmaster fill valve by gripping the valve "riser" (to keep it from turning) and rotate the upper valve housing CCW 1/4 turn, to open and tip it away from the riser, exposing the washer/diaphragm. Barely crack the supply valve open a bit to allow water to flow up the riser to flush debris out, using the flowing water to bathe the washer/diaphragm and flush away debris caused by your maintenance activities.

Reassemble and live in peace.