could be a lot of things. salt for example from possibly some oils they added as it dried out the salts migrate to the surface and appeare as a dry powder. Bruce has a guess just the rest of us did his method work? he told you how to deal with it. it takes minutes to find out?. That leather product is ruined period. Not all leather is salvageable nor does it need to be. Not all leather tools are salvageable not all belts, saddles, and boots in the world can be fixed that is a simple reality. If you do get out the unknown stuff safely you still have a withered and dried-out piece of leather if you get the oiles back into it it will still be an oiled damaged piece of leather that needs to be re-sewn to be any use on a horse carrying goods. If you ruined your tack deal with it move on and don't do it again. Frankly, if you brought those in my shop unsealed I would kick your butt right out the door and scold you for the damage you did to them.
I used to post and help folks a lot here. I don't much now because frankly, most folks who post these types of questions want the answer they want not really a truthful response, and when they don't get that then whomever is the bad mean person. I do not come on here to blow smoke up people's butt I come on here and attempt to help by giving truthful honest answers to the pictures and few words folks try to communicate with their mostly limited knowledge of their problem. You have a great life and I wish the best for you and sincerely hope you can post an I told you so pic of those fine bags made all better.