Does Counterspell work against a creature’s innate spell-like ability?

@GSLLC @JeremyECrawford#DnD #5e question: Does the spell, Counterspell, work against a creature’s innate spell-like ability? works against anything that is a spell. so, a creature that has an ability to cast fireball would be subject to it

— Mike Mearls (@mikemearls) April 25, 2015

@GSLLC @JeremyECrawfordBut Medusa’s gaze. “Magical” but not a spell, so can’t be counterspelled, right? Drow innate spell could be.
correct – ability must boil down to “creature casts X” where X is a spell

— Mike Mearls (@mikemearls) April 25, 2015