MACMAC This is curling, the edge of the newly printed layers are curling up away from the bed. It is pretty common with unfilled materials. More cooling and slower printing will reduce it. Also make sure you are only using three walls, more walls will exacerbate the problem.
With PLA prints this is a common defect that is hard to cure on any narrow overhang, I have it on several of my prints. Minor aesthetic issue.
