Why Variegated Leaves Turn Green
Leaves of variegated plants occasionally lose their colorful markings and return to plain green. This twist of nature can be frustrating when we spend extra money for the unique foliage markings.
Some variegated plant introductions are derived from a portion of the plant that has mutated. In this case a branch or offset of the plant produced variegated leaves. The mutated portion of the plant is propagated by division, cuttings or tissue culture to maintain this unique quality.
If the variegated leaves undergo a reverse mutation, the leaves change back to the original solid green leaves. It is also possible for the plant to undergo another mutation, resulting in new growth that looks different from the original and purchased plant.
There is nothing you can do to stop or change it back. Just remove the plants or the parts you don't like.
A bit more information: Offspring of variegated hybrid plants may fail to retain their parent’s variegation. Seedlings of hybrid plants often look different than the parent. Leaf cuttings of some plants like a variegated Sanseveria, snake plant, lose their variegation while divisions will maintain this characteristic.