Hydromel is a vaccine used by the Vanir to stave off Lazar's Disease. Hydromel is an organic compound, green and phosphorescent in appearance, which is inserted in the Vanir's armour to protect them from infection from the disease. The Vanir need constant supplies in order to replace the depleted Hydromel, which becomes yellow and loses its phosphorescence when used up.
The corporation that the Vanir are enslaved by use the Hydromel in order to keep them working at Terminus, since they would cut off their supplies if they stopped obeying. (DW: Terminus)