Looks like porphyrin to me as well! Sometimes one of my rats Edmond gets lazy about cleaning his face and let it build up a bit like this around his nose. It’s not even an indication that he’s stressed or sick, just that he hasn’t wiped his nose in a little while. Usually his brother will help him clean it up if he’s missed any spots. Especially if he’s been napping, this happens to my little guy. Maybe wash their hammock or wipe down the ledge they’re napping on, because its sometimes an indication that they’ve been napping in their own pee.