Eye problems usually occur for 3 reasons.
1. The dog has an eye infection. If the mucus is green or yellow, you can assume that this is the cause.
2. If the dog has allergies, the eyes might produce more tears.
3. If the dog has suffered a trauma to the eye, you might see this as well.
You should clean the eye with warm saline and dry it well with a clean dry towel. If the gunk returns, you need to see the vet tomorrow to find out what you are dealing with.
Let me know if you still have questions.
