I had this same problem with my order last week. Only a few of the 25 had the problem though.
I used a warm wet papertowel to get the crusty stuff off, then used a qtip and put olive oil around the vent. I read in another post that this would keep the poop from sticking as much.
It seemed to work for me, but i did have to apply it for a couple days.
They say that giving them yogurt also helps (mine wouldnt eat it), because there is a certain bacteria they are missing that causes the pasty butt. So you may give that a try as well.