Mostly I see that with harness leather. Harness leather has been impregnated with extra tallows, waxes and greases to make it last while in constant use against a sweaty horse. I have a knife to make a sheath for right now in the shop. It was sent to me in a scout style sheath of harness leather. Just sitting around waiting its turn some of that white stuff has surfaced on the scout sheath. If its harness leather a little warmth will usually melt that stuff back into the leather. Even just the friction of rubbing with a rag is sometimes all it takes. If a little more heat is needed try a hair dryer on low heat and then rub. This is real common on pack saddles that sit for a few years. It just happens On these I oil em/condition em back up and put them into service. it goes away with use.
That being said I once had some veg tan that did that too. It was suppose to be a "premium" leather by a well known tannery. This was a little different it was white haze/glaze on the leather after working. I called my supplier, they replaced it and then called back after I sent the lether back to them. They'd experimented with it in their own shop and had the same problem. It seemed to have something to do with casing and getting the leather wet. In that instance there really wasn't anything that I could think of doing. This white haze wouldn't wipe off, heat didn't help. I could get rid of it about 90 percent by using 2 or 3 coats of BagKote and rubbing it very hard but that was a partial cure, didn't get it 100 percent. Best guess was QC at the tannery Quein Sabe?
Hope this helps. I do know the frustration of working with leather that goes south on ya for some reason or other.