It very well could be retained shed in the nostril, but it's impossible to tell without looking at the snake. If it is indeed retained shed, you could try a 15 minute soak in luke-warm water, then CAREFULLY use a pair of tweezers to remove the offending piece of skin... If you're not comfortable doing that, you should be ok to wait until the next shed. Just make sure to up the humidity to 60-70% so you get a good, full shed next time...