under eye discoloration

My JRT is 10 and is the best dog ever, when he's not chasing one of the neighborhood cats. He's all white. For several weeks he's had discoloration forming under his left eye. At first I thought he got dirt or sand under his bottom eye lid and washed his eye out twice a day but that did not work. I’m noticing it's wet under his eye like something’s coming out, but there is nothing visible like green puss. I looked under the lids, both top and bottom and both right and left and they look exactly the same, a healthy color pink, not red or irritated. Then I thought he ran into something doing 100 mph and he had a black eye, but it's not going away. He acts fine, he eats fine, he's drinking water fine, he walks with a bounce in his step and we walk 3 miles every night. I can't figure this out and I hate to spend $200 at a vet when it's a crap shoot with them as well. Any suggestions??