I apologize for the delay in responding to your question.
If your kitten has been given it's complete set of vaccinations to date and has been otherwise healthy, it is possible he is simply suffering from a gastric upset condition. Not that this isn't a need for concern, because mucous can mean infection, but if vaccinated and at 6 month's, the chances lessen that is something critical. His symptoms may also mean a worm or parasitic problem.
I'm very glad to hear he is drinking water - that is very important and he needs to keep fluids in him to prevent dehyrdation. I suggest you feed him a bland diet of boiled rice and boiled chicken; or plain chicken baby food. Be sure to feed him in small amounts - and frequently throughout the next 2-3 days. This diet is tasty to most pets and will also help him keep his strength. If he should vomit after eating, wait an hour, reduce the amount of food, and try again.
If you haven't done so, I suggest you find out where the nearest emergency vet clinic is located just in case his symptoms should worsen.
If he is stable and will eat and continue to drink through the weekend, contact your vet first thing Monday morning because he should be examined.
All the best and I hope he feels better very soon,