We rescued little Nova from a colony of cats living in a junk yard and we noticed that she had some black crusty looking stuff at the edge of her nostrils. I thought it was probably just dirt and that it would come off when she cleaned herself or during her flea bath. It didn't so I dipped a q-tip in a little bit of hydrogen peroxide and gently swabbed her nose until it came off, which was difficult. A day or two later I noticed she had the black crusty stuff at the edge of her nostrils again. I cleaned her nose a second time. Now the crusty stuff is back again. What could it be?