This unnamed red liquid, often known as acid, is a hazard that only appears 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 heavily damage Mega Man if he makes contact with either the drops or the ceiling pools, so the player must avoid these as much as possible.
Due to the drops being red in color, the acid drops seem to resemble blood.
Acid was present as a threat in the second episode of the Mega Man cartoon show, where a spy sent by Dr. Wily traps Mega Man in the bathroom, and floods the entire room with acid in an attempt to dissolve him. This is likely coincidental.