I don't know what it is, but I would suggest you pot it up in soil.

Use a potting mix not garden soil and use a plant pot with drainage holes, not a jam jar. This applies to whatever plant it turns out to be.

I suspect it's some kind of Hoya.

I agree w/ suggestion it needs a proper pot w/ mix & a hole in the bottom. Takes medium sun, likes bright, indirect light.

The one in the jar, I'd cut it at the stem just above that whitish looking stuff & sprinkle it w/ cinnamon, the cut end (acts as an anti-fungal, preventing rot). Then until you get a proper pot, pls. put this in fresh, clean water (or it'll keep rotting 'til it dies.)

Do you recall if it ever bloomed for Grandma? Condolences on her passing, nice to keep a plant that was hers.

Gradually increase light after repotting.

And give it a bit of fertilizer. The timed-release fertilizers like Osmacote are easy and can be found at the big-box stores (WM, Lowe's, HD, etc.).