dnd 5e – Mage slayer gives disadvantage on concentration on all attacks or just on the reactive one?

Each bullet point of mage slayer (PHB, pg. 168) is a separate ability:

  • When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.
  • 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 have advantage on saving throws against spells cast by creatures within 5 feet of you.

As long as you attack a creature who is concentrating on a spell, you can force disadvantage on their saving throw for concentration. The prior bullet point simply grants you reaction attack if a creature next to you casts a spell. Although if you hit, then they would have disadvantage on their concentration save per the second bullet point.