Does triggering an effect of a spell count as casting a spell?

Spells like Moonbeam, Flaming Sphere, and Call Lightning allows you to use either an Action or a Bonus Action (depending on the spell) to trigger a specific effect on subsequent turns. Does triggering these effects count as casting a spell?

I’m fairly sure that one can’t Counterspell them once it has been casted, but would it trigger the Mage Slayer feat, for example? (Assuming all conditions for Mage Slayer are met)