Hi and welcome!
Is the crust watery-clear or is it opaque and yellowish/greenish? Or is it is crusty skin?
Unlike humans, guinea pigs can't breathe through their mouths, so any obstruction somewhere in the airways is always very audible. If the sneezing is watery, then it is very likely that it is from an irritation in the nose. If it does pass in the next few hours or if the crust is thick and yellowish, then please have him seen by a vet. In that case, there is a chance that you are dealing with a bacterial respiratory infection (URI), which needs an antibiotic to cure. If in doubt, please see a vet, as URI can kill if left untreated/undertreated.
In the third case, we are talking cheilitis (lip infection) if is is on the lips or fungal if is it on the skin of the nose.
Since we have got members from all over the world, we find it very helpful in being able to tailoring any advice to what is really available and relevant where you are if you please added your country, state/province or UK county to your details. Click on your username on the top bar, then go to personal details and scroll down to location. Thank you!
If you are worried about having to take your guinea pig to the vet, you may find this guide here helpful. It contains tips on how you can ease the stress.
Tips For Vet Visits