The leech is the alien's mining structure. Like a mushroom extracts nutrients from the ground, this similiarly shaped creature sucks some invisible resource right out of the air (it competes with the human mining structure). It can be built on walls and ceiling in addition to the floor and thus needs to roughly fit into a cube for technical reasons.
Original concept by Khaoz (note that the proportions aren't ideal as the bounding box doesn't describe a cube):
Previous model attempty by kuntiss:
If you chose to look at kuntiss' version, you can use similiar proportions if you like (note how the bounding box describes a cube). Apart from that feel free to create your own interpretation of an alien fungus sucking valuable nutrients out of the air. It doesn't need to look like a real mushroom at all.
Animations should include idling, pain and death and they should look fitting regardless of the orientation in which the structure is built. You can animate yourself or have us assign this to a specialist.