Invincible Flesh Larva in Mud Cave / ACT 1

While exploring the Mud Cave in Act 1, I came across a rare spawn for a larva. As larva normally start by coming out of the ground, the rare spawn was surrounded by these spawns, however, these grubs in particular came out of their spawn before I could even activate the rare spawn and began attacking me. These larva were invincible and only were killable once I activated the rare spawn.