Limax

From Ben10versepedia

The Limax are shape-shifting slime aliens from an unknown world. The Limax thrive in conditions of fire and heat, hence why they chose to begin their invasion plans in a retirement community situated in the middle of a desert in the summer months. They come to Earth to collect elderly people and devour them; they indicate that young people don't taste as good. The Limax seem to have a hive mind, being able to coordinate throughout a large area without any direct contact, though they do speak to each other. The Limax can merge with each other to form one gigantic Limax. They melt when exposed to cold water, which evidently is poisonous to them.

After their plan to kidnap Aunt Vera's retirement community failed, most of them escaped on their spaceship. A few, however, were left behind. These aliens appear in the episode "Permanent Retirement" and are mentioned in the episode "Perfect Day" when Enoch in the form of Lt. Steel claims that the real Gwendolyn Tennyson and Grandpa Maxwell Tennyson are Limaxes in disguise.