Try applying a warm (not too warm!) compress to soften the gunk. If it’s green or yellow and the eye looks red or irritated, take him to a vet. I hear blocked ducts is quiet common in bunnies and requires vet care.

I agree with Beau that if the fluid from the eye seems like a concerning amount (I would say more than what you’d expect from a human who wakes up every day with those sleep crusties), then a vet assessment can be very beneficial.

A warm, damp towel to help loosen it is fine, especially if it’s something your rabbit cannot get himself. Other than that, leave it to him since he’s probably the master at cleaning his entire body, haha.

To go along with everybody else, you should consider taking your bunny to the vet as eye goop is often a sign that something else is going on. In my case some years ago, I had a bunny with eye goop that was creamy colored, then yellow and even green before I went to the vet. Trust me, don’t wait on this because it won’t go away on its own and it doesn’t sound like the harmless eye crusties from sleep.

If you have a fine tooth flea comb, that can help remove more  once it’s softened.