Obsidian Grubs are passive creatures that live in small packs from 1-5. When attacked or startled, they retreat into their shell which makes them slightly harder to eliminate. It's advised to ignore them, as they provide little food and are passive both to Monster and Hunter. The only incentive one may have to kill an Obsidian Grub is to gain a perk if the chance arises that they are an Albino.
The Albino version gives Hunters a 75% clip size increase while the Monster will no longer startle birds.