Movie film Q: what is this crud on the edge of my film?

OK -

Just bought a couple of films, and while trying to clean them, found a white encrustation along the perf edge - I don’t want to try to remove it with force (the film is over 30 years old, and rather scarce), but it won’t budge with film cleaner.
Any ideas? This is NOT nitrate stock, and there is no hint of vinegar syndrome - it looks kinds like little bits of dried salt - rough texture.

Luckily, it does not interfere with the image of sound, and the perfs are good.

Since it’s only along the perf edge, my thought is that it’s some sort of dried up lubricant, or the result of a reaction between a lubricant and the film. WAG%=58.