The effect to reduce the ATK of monsters with A-Counters is an effect of this card. If this card (or another with the same effect) is not on the field, then a monster with an A-Counter will not lose ATK while battling an "Alien".
If 2 "Alien Grey" (or other monsters with the A-Counter text) are on the field, then a monster with 1 A-Counter will lose 600 ATK while battling an "Alien".