Photo is not clear enough to be certain, but the whitish marks could be just water stains. It could be something else thou - maybe powdery mildew? some kind of fungi? Try to take a clear photo of it.
Plant looks very etoliated (stretching for light), also very overcrowded.
I would take it all out of the pot, clear all of the soil off the roots, inspect the roots for any damage and/or signs of rotting or drying up. Cut dead and damaged roots off. Get a good potting mix and repot - don't put them all together again. Many ppl prefer just a single plant per pot. Nothing wrong with potting few together, but not so many.
Btw, on Cacti & Succulents forum there is hundreds of discussions about jade plants...Rina

Jades are not hard to please. Here's what I would do:

Step 1. Give it a big drink of water (go ahead and try to rinse the leaves off to see if that gets rid of the residue).

Step 2. Place it in a VERY sunny window - ideally an unobstructed southern facing window. The more light, the better for a Jade.

Step 3. After it has had some time to regain some energy from the better lighting conditions, unpot it, gently divide it into several pieces (with roots attached to each piece) and let the pieces dry in the open air for 24 hours. Letting any wounds on the plant dry out a little before repotting will reduce the risk of rotting.

Step 4. Get some light, airy, well-draining soil and repot the plant pieces into several different pots.

Step 5. Place it back in a sunny window. Water when the soil dries out.

Jade plants are quite susceptible to a fungal disorder called powdery mildew. It's impossible to tell if that's the problem from the poor quality of the image, but judging from the overall poor condition and your description of the leaves, I'd say it's a good guess.

UUntreated PM can kill leaves.

Is it a Crassula arborescens really? They sometimes have a white covering on the leaves, depending on the variety.

I'm really not sure which variety it is sorry!