What are your water parameters?
Mollies do not belong in 15g aquariums, they get quite large and are quite active. The problems that come with having inadequate stocks may be a reason why he passed. How are your other fish doing? Do you have a larger tank for the other molly? If not, please consider removing the molly and if you want something similar, you can have platties instead.
Also, cories should be in groups of at least 4. They really need buddies to thrive and show natural behavior. You can have a group of 4 panda cories or juliis, but I wouldnt add more, and other species will get too big for the footprint of your tank to have a group of them. You can also trade in your cory and buy pygmy cories. You could get a larger group of those.
What is most important right now is knowing what your water parameters are so that you do not lose any more fish.