Dry Skin under armpits? Normal or something that I need to treat? HELP Please

[Dean21]

Hi all, I noticed that one of my chickens had some yellow dry flakes (about the size of raisons) underneath the both armpits. It looks like dandruff, but it was yellow. I gave him a warm bath and slowly removed most of it gently. However, then I decided to check the other chicks (all about 4-8 weeks old). About 30% of them have it (5 of our 14 chicks) What do you think it is? It is only under the armpit, nothing on the faces, legs or other body parts. Here are pictures.

I guess my questions are:
1) should i worry?
2) should i separate them from the others?
3) what's the treatment (if any)?

Your advice and guidance is much appreciated.

 

[Eggcessive]

Most of it looks like flaking old skin except for the 10th and 11th pics, which might be a scrape healing. Some chickens are not very good about dust bathing, so make sure they have an area for that. pictures like these have been posted in the past with silkies being common.