EAGLE - StopMask DRC Error
It's due to the fact that your reference designator font is "proportional", not "vector".

Proportional fonts in eagle are collision detected with a rectangular box which is what the DRC is erroring about - the bounding box of the ref-des intersects the stop mask.

Vector font converts to a series of lines so collision detects the actual lines rather than the bounding box.

I'd recommend always using vector font. In fact there is a setting in the layout editor options for converting all fonts to vector.