Your description is quite vague; a picture would be much more helpful in this case. It may be algae on your Swords (which are small now, but will outgrow your 10 gallon aquarium very quickly).
Melting is not triggered directly by low light conditions per se, but can be triggered when plants are moved from one environment to another, different, environment. Some plants, such as Cryptocoryne spp. will sometimes melt even when moved within the same aquarium.
Melting looks like the plant is disintegrating, and turning to clear mush.