Black Slime (5e Hazard)
From D&D Wiki
Black slime can be found coating water surfaces like an oil slick, moving only where the currents take it.
A patch of black slime covers a 5-foot square, and can form strands that can dangle 10 feet below the surface of the water. A creature coming into contact with the slime or its strands 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.
Black slime only absorbs living matter, so it can be scraped off without damaging a weapon or tool. However, scraping while swimming is a harder task, and black slime scraped off in water has a tendency to float right back onto its victim. Thus scraping is only successful if the victim succeeds on a DC 10 Dexterity check.
Unlike its green slime cousin, black slime is not affected by sunlight or cold. Any effect that cures disease destroys a patch of black slime. Any effect that deals fire damage also destroys a patch of black slime, and it continues to burn as though it were oil.