These large creatures tend to appear near any humanoid activity in the mud oceans. Called "Imprints" for the ability to reflect dreams and fears of the colonists.
It is a combination of 3D Coat sculpt and liquid sim done with Maya's Bifrost. Though still there was a lot of hand tweaking of the simulated and original meshes.
Based on Fotoref's photopack: https://www.fotoref.com/products/mud-pools
I also used book from my home library that is a superb reference for understanding effects: Elemental Magic II https://goo.gl/rUrStb (I have both volumes and I think second one is better)