My two orange cats are prone to getting eye boogers and nose crud. Vet told me its like PatandChickens said, low level upper respitory symptom, it can be due to numerous things but most commonly just seasonal/allergy related. Similar to how we get stuffy or runny noses in spring and fall. For some reason I have always noticed light colored cats (white, orange/yellow) are more prone to getting it than dark colored cats, and its not just the color difference, I check all my cats regularly and rarely do my black or grey/blue cats get eye boogers or nose crud, but my eldest orange cat (3 yrs old) has the stuff all the time right now. Ugh.
If the stuffs been dried on the nose for a few days its going to hurt when you try to get it off of him/her...Moisten it up with water, neosporin, or vasoline (someone else already suggested this too ) and gently rub it off. If you try to peel or flake it off dry (if you don't check every day) it can peel off the sensitive skin or hair around the nose with it. And obviously thats going to hurt.
If kitty's nose stays warm and dry and they keep getting the nose crud I would suggest a routine vet visit to make sure its just seasonal, as it could be other issues, but it doesn't really sound like it.
Good luck pickin' them boogers