With dogs who have tearing eyes (the dark crustiness under the eyes), it is important to keep the hair under their eyes trimmed short to allow easy cleaning. It is not unusual to see dogs with sore areas and skin infections under the eye area.
Dog tears and saliva contain an enzyme that causes a chemical reaction on the hair turning it red and eventually brown and the tears tend to cake up . There are many products on the market to remove this. They are marketed as Dog tear Stain Removers.
The website below has a different approach suggesting altering your dog's pH levels to prevent the staining which you may wish to look at.
it seems like the constant build up has allowed the area to stay wet and your dog may have developed a yeast infection of the skin under the eye. This will cause the sores you are seeing.. You will want to clean the area real good, being very careful of the eyes while you are doing this. Trim the hair back and try some miconazole cream which may provide some relief until you can have her seen by your Vet.
Hope this helps.
Thanks for the additional information. You may be seeing oral papilloma if this is more growth like which are warts. In most cases we see them in the gum area or between the toes of a dog but they can be around the eyes as well. It usually clears up on its own in a few weeks. You can read more about this here:
However, if it is starting with a crustiness, then it still might be that some normal eye discharge is trapping moisture under it when it dries and thus is leading to the sores. One thing you might want to try is an antibiotic eye ointment such as Terramycin Ophthalmic Ointment. If there is any infection in the eye area, this should help clear that up.
Another thing that may help prevent this if it isn't papilloma, is using Benzoyl Peroxide wipes on the area on a regular basis to keep bacteria down and help prevent the "crusting".
This may be something that requires a hands on examination to determine what is happening especially if there is any redness to the eye itself or colored discharge.