It looks like a stain. I can barely see hatch marks in part of it. What's the story? Did it get moisture in it after it was assembled? If it doesn't have any depth to it, I'd try running that piston all the down in the hole, put a rag in there (to trap the fluid) and try scrubbing it with lime or lemon juice (citric acid) and an old toothbrush and see if it goes away. Otherwise, if there's depth it's the next size bigger (be sure to sonic check it)!