These pods are the Chrysalis, the birthing chambers of the Swarm Cast. Inside their delicate membrane is a green glowing fluid inside of which the zygote of a swarm creature rapidly grows.
It sits upon a bed of slime called Processing Bile which breaks down organic and inorganic material and makes it usable by the swarm.

These pods are where new Swarm creatures of the Cast race are born. When they are psionically activated by a Queen or Brood Keeper they are given a template, the primordial fluid in the pod takes that template and begins forming the new creature along that blueprint. A new creature is grown and ready to fight within fifteen minutes of activation.

The fluid contained within is the primordial substance of the Swarm Cast and highly infectious to any creature not already a member of the Cast race. Those who come into skin contact with the fluid are immediately infested, and it can take an hour or up to a whole day for the creature to become fully infested and converted into a swarm creature.

This is the tool used by the swarm since their creation to reproduce and churn out monster after monster as if from a factory line.
It's not very sturdy or protective, so it must be protected by others for it to survive.

The Chrysalis, like all swarm tools and constructs, can only grow on a bed of Processing Bile.

Swarm - The Birth of a Race