Cryptocoryne sp. (crypts) is a sensitive plant from tropical regions in south east Asia. In their natural habitats, crypts mostly grow in rivers and streams. and inundated seasonal pools.
Many crypts species respond to environmental changes such water parameter changes with what is commonly known as Cryptocoryne melt. When a established crypt of these species experience a change in its environment, its leaves rot or "melt" but its root system stay healthy. Eventually the plant grows new leaves that are adapted to the new environment and the plant thrives again.
In the planted aquarium hobby crypt melt is usually triggered when a plant keeper gets a new crypt (e.g. purchased or obtained from other hobbyist) and plants it in his or her aquarium, the plant will usually experience a melt if its coming from a very different growing condition.
In the commercial market crypts are mostly cultivated as emersed plants. When emersed grown crypts are submerged in aquarium, they will almost always "melt" their emersed leaves and grow new submerged leaf form.
A established crypt can also experience a "melt" in the same aquarium if there are any major changes in the tank parameters such as water parameters, temperature, lighting condition, and CO2 or nutrient availability.
With proper nutrition and a healthy growing environment, it will revive and grow new leaves in a few weeks.
Happy Cryptocoryne growing!