Crusty black gunk around nose

I took in a pregnant stray a couple of months ago. Goldy's kittens are 5-1/2 weeks old now. It's not a daily occurrence, but every once in a while they get a crusty black substance along the edge of their noses. It does not appear to be inside the nose and does not block their breathing. I can wipe it off easily with a damp cotton pad. Goldy has had the same thing a few times as well. My other two cats have never had anything like this.

Any ideas what it might be? I should also add that I have not seen any sneezing or runny noses.

I think it's boogers...My cat Holly has that but not Buster.

As long as it's not some kind of parasite, I don't really mind wiping their noses once in a while.

Kiwi has that all the time. I just wipe it away with a wet Q-Tip. My vet said it is just cat boogers, and since Kiwi's face is white it is more noticable than on a cat with dark fur.