Melt, Its a term used to describe what happens to some aquatic plants under stress or experiances sudden condition changes.
Plants tend to turn brown, go mushy and loose all foliage.
Most common in Cryptocoryne species, but also occurs in java fern and other aquatic plants.
Excerpt taken from wikipedia
'Crypt melt'
A phenomenon often encountered when planting new crypts in an aquarium is commonly called Crypt melt, whereby the plant loses all its leaves. There seem to be two possible causes for this.
Rapid environmental changes is thought to trigger this, as these plants don't seem to adapt well to transplantion, and may need thirty days or so to become established and for the leaves to regrow. Experienced growers report that it is better to plant crypts in aquariums that have been established for at least three months