That's an electrolyte material deposited from the battery that's dried up, left in powder form. I found my old high school T-85 graphing calculator a few years ago sitting in a box. It takes 4x AAA's. I tried turning it on, but no power. So I flipped it over and popped the back battery door open. The entire inside was coated and cemented in with that white powder. I'd experienced this earlier in life several times with flashlights, remote controlled cars, toys, etc., and learned to take batteries out of the stuff I wasn't using. I'd been using my graphing calculator so much in the past few years that in my head, I didn't think I'd ever stop using it. I did. The white powder will turn green if it sits, leaking for a suuuuper long time. This is what Panasonic says about it:
View attachment 962181
View attachment 962179
So yeah, removal of the batteries in unused mods is highly advised for the sake of the mod and the batteries. I will however mention, as you have Junkle, that it won't necessarily render the mod or whatever product this happens in useless, but it certainly isn't good for it. I'd also keep an eye on the condition of the contacts and any other metals inside or near the battery harness.