Mage slayer gives disadvantage on concetration on all attacks or just on the reactive one?

Mage Slayer:

When a creature within 5 ft. of you casts a spell, you can use your reaction to make a melee weapon attack against that creature, and when you damage a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration. You also have advantage on saving throws against spells cast by creatures within 5 ft. of you.

Does that mean that when this PJ hits a mage concentrating on a spell with the Attack action on their turn, they also have disadvantage? Or does it just happen when the attack occurs upon being triggered by a spell being cast on mele range?