If this spot is in fact sand that is somehow stuck in the eye, you really have a challenge here. You really don't want to touch the eye with anything since you'd most likely do more harm than good. My thought would be to use a turkey baster filled with water to try and squirt or flush the sand out. Its going to be very tricky to get that close to the fish while its in your tank but I would try whatever means possible without taking the fish out of the tank. Its very stressful for the fish to be netted and put in a temp. container. More importantly if it is sand, the fish can damage its eye even more durring netting while it thrashes around in the net. If the flushing doesn't work, I would recommend you consider leaving the spot the way it is, in particular since it doesn't seem to bother your fish. Good luck