My son has some yellowish greenish goop coming out of one of his eyes. What is this?

My two year old son has yellow/green goop coming from the corner of his eye (next to his nose). It doesnt seem to be bothering him. The eye itself looks fine. All day, another strand of goop comes out and it looks like he just woke up. Should I take him in? Is there some kind of over the counter drops/ointment we can use? Thanks!