It looks like it has end stage freshwater ick. That is VERY difficult to ID on Dalmatian mollies due to their black and white color, but I think that's what it is.
Sorry, but I don't think you will be able to start a cure fast enough to save it. If you want to try, you need to bring the salt level up to 4 parts per thousand. The amount to add is 0.39 pounds per ten gallons. Do this is two additions, a few hours apart. You need to know exactly how much water your tank holds. At the same time, you need to start a freshwater ich medication. There are a lot of junk products out there, so see if your pet store has any good ones they recommend. Ich Guard is one that works.
Jay