Alien Creeper is a mob added by the Falling Meteors mod. Alien Creepers are hyper-charged creepers from space, and generally arrive when meteorites fall and hit the overworld surface. They also seem to spawn randomly while mining meteorite ore, making meteorite mining very dangerous. Be sure to have a good escape route. The explosion radius is 4-5 blocks larger than that of a normal creeper.
When you kill it, you have a chance to get a red meteor gem. Alien creepers seem to have the ability to pull the player towards them before detonation, making them especially dangerous to encounter. Thus, it is best to attack them with a bow.
Alien Creepers appear as purple creepers with orange speckles and antennae on their heads. They are always surrounded by the blue glow texture of a charged creeper.