What are your water parameters?
Is it possible that the scales have been lost due to an injury? Or from any other fish, maybe the other molly or a
Platy, harassing him? Are the scales mising clean or is ther fungus growing also?
I would be inclined not to medicate. Especially if they have been lost due to injury or in a scrap with another fish then all it needs to heal is good water conditions, therefore plenty of water changes. you can add some melafix if you like but it's not necessary.
If there is fungus growing then again this could simply be secondary to an injury, but it may need to be loked into further.
12g is a bit small for mollies and platies. The mollies especially can grow quite large (4") and really 30g is a better minimum tank size for them.