While searching for an answer i found this and this question about counterspelling a Mind Flayer. Which does not work because Counterspell states in it’s reaction trigger:
when you see a creature within 60 feet casting a spell
and the Mind Flayers Innate Spellcasting tells us:
It can cast the following spells, requiring no components
So we know, Counterspell does not work because a Mind Flayers spellcasting is not observable.
However Mage Slayer reads like this:
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.
RAW this seems to mean, that a Mind Flayer triggers that melee weapon attack because Mage Slayer does not require you to see the spellcasting. This feels wrong to me because there is no way to even tell the mind flayer casted a spell except the spells effect which might be observable (levitate) or not (detect thoughts).
Is there some kind of ruling, that i am missing?