pick at

Short definition from Wiktionary

Verb

1. 1
   To touch, grab, handle, or pull tentatively or gingerly, using a utensil or one's fingers.
2. 2
   To touch, grab, handle, or pull tentatively or gingerly, using a utensil or one's fingers: Synonym of peck at.
3. 3
   To pick on or repeatedly criticize (someone).