Hi Karen,
The feather which is in his nostril is likely simply stuck there because it was floating in the air. You can safely remove that if it is still stuck on his little beak/nostrils.
Yes, finches do have nostrils. They are just at the top of the beak area.
If you finch looks even a little like this:
Treatment for these burrowing mites is best performed by a certified avian vet (http://www.aav.org/ to help locate one near you). This is because the medication that can kill the mites will also kill the bird if used improperly. Finches are sooo tiny, you can easily see why caution and medical advice is the way to go when treating the cute little birds.
The good news is that you didn't cause this and once you get it cured and the cage(s), food dishes, toys, etc. steralized it shouldn't return again.
The bad news is that if you do not treat it, the birds suffer very badly from itching since they feel the mites burrowing under their skin, into their beak, and the spread to the feet. Here's a budgie that wasn't treated for a long time: (found at http://burgebirdservices.homestead.com/ScalyFaceS.jpg)
A lot of people will tell you lots of home remedies. Pet stores will try to sell you products to treat these mites. Please do not fall for it because they usually do not cure the problem. At best, these products might kill some surface mites, but the internal ones and the eggs will continue the cycle and you can kill your birds with the products if used incorrectly.
Here are some links to more info on burrowing mites below.
If I can help further, please use "reply" after accepting this answer.
Several links to Scaly Face articles: http://www.finchworld.com/health/scaly_beak.htm
Discussion on Finch Scaly Face: http://www.tailfeathersnetwork.com/community/showthread.php?t=35252
Scaly Face: http://www.birds-online.de/gesundheit/gesparasiten/grabmilben_en.htm
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