Depends on the conditions in the tank. If the aquarium contains soft, acidic water then removing the salt is bad, and the mollies will, sooner or later, get fungus or fin-rot. If the aquarium contains hard, alkaline water however, you will probably be fine.
It's a shame mollies are so widely sold without warning people what they need. Unlike most other tropical freshwater fish, they do not like soft, acid water and really should NOT be kept with neons, angels, rams, discus, Corydoras, loaches, sharks, gouramis, etc., etc...
Cheers,
Neale
PS. The plural of "molly" is "mollies", not "mollys".