They have 3 HP
They show up as a small flat green spot on the ground and attack the player when stepped on after a short delay causing 5-15 damage. They can detect the player even through stealth.
The easiest and safest way to deal with a lurker is to run directly over and past it, then turn around and attack it before its attack animation ends.
Should your melee weapon be too slow, switching to your fists will allow a fast enough attack.