I have a A23 Pro and A29. Both have similar imperfections. It is most likely the excess of lens material from the flange of the lens. This flange is sandwich between the head shell and the led housing. There isn't any o-ring between these parts. They seal it by just screwing it down together and since the flange is acrylic, its just excess material.
Its just a matter of dismantling the led housing, remove debris and reassembly. Best if you have circlip pliers.