From Wikitroid, the Metroid wiki
A Red X.
are red-colored
X Parasites. When
Samus touches them, they restore 4 energy tanks worth of health, 10
Power Bombs and 50
Missile ammunition. Red X usually only come out of
Bio-Doors and certain variations of normal enemies (usually of a different color and harder to defeat), although they can also rarely appear from normal enemies. They are the least common X.
