It could be a normal protective mucus, which when you remove it comes out as a transparent very fine and in a string. I have this from time to time, the cause being an eyelash on the top eye-lid which instead of turning up as it should, turns inward, it is hardly noticeable, but it irritates the eye-ball, which causes the mucus to form. When this happens, I pop into the optician who plucks out the offending eye-lash, and then it's fine for quite a few months. Trouble is the eye-lash eventually grows back again, so it's off to the optician again. Perhaps you have a similar situation.