I agree with Mike insomuch as that without acclimation going straight from fresh to salt is cruel and would hurt any fish.
I further agree that the flow rates in the average reef tank may be too high for mollies, and that certain reef fish such as damsels, pseudocromides and the like may be too agresssive for a poor little mollie to cope with.
However given the correct conditions Mollies are in fact happiest in full strength seawater, far happier than total freshwater ... I have reviewed the literature extensively, and I fail to see evidence as to the practice of mollies being kept in saltwater being cruel given the correct acclimation process and tank conditions.
Keeping mollies in a sump tho with tiny compartments, and then subjecting them to high reef flow rates and aggressive reef inmates would be cruel tho.. So if you do choose to do this please ensure you turn down your pumps beforehand and have no agressive tankmates.
Further I think that the practice of feeding live mollies to lionfish is cruel, however I don't think that was Mandarinmans intention.
Finally I must say that it's probably a far better Idea to solve your algae problems at the source... with say an algae scrubber (work very well). Keeping the mollies in the sump involves extra feeding and this would cause extra nutrients to be feeding the algae growth in the display, which you are then going to use the mollies to eat... could work but I feel that using hermits & snails may be more efficient combined with a nutrient reduction program.