Green Slime

From Dungeon Master's Guide, page 105.

Hazard

This acidic slime devours flesh, organic material, and metal on contact. Bright green, wet, and sticky, it clings to walls, floors, and ceilings in patches.

A patch of green slime covers a 5-foot square, has blindsight out to a range of 30 feet, and drops from walls and ceilings when it detects movement below it. Beyond that, it has no ability to move. A creature aware of the slime's presence can avoid being struck by it with a successful DC 10 Dexterity saving throw. Otherwise, the slime can't be avoided as it drops.

A creature that comes into contact with green slime takes 5 (1d10) acid damage. The creature takes the damage again at the start of each of its turns until the slime is scraped off or destroyed. Against wood or metal, green slime deals 11 (2d10) acid damage each round, and any nonmagical wood or metal weapon or tool used to scrape off the slime is effectively destroyed.

Sunlight, any effect that cures disease, and any effect that deals cold, fire, or radiant damage destroys a patch of green slime.