Red substance

"Acid" is the hazard that appears only in the sixth and final Wily Castle stage in Mega Man 2. It appears as a large, red pool of liquid that drips from the ceiling. Many of these pools of liquid are seen all throughout the narrow passageways, and can damage Mega Man heavily if he makes contact with it, so the player must avoid these as much as possible.

In Mega Man: The Wily Wars, these pools of liquid also appear in the fourth and final stage of the Wily Tower.

It might be blood.