Freezing Mark (Marks a target...) cannot be applied to ice crystals, which are a target...

Freezing Mark says,

"

Marks a target, making them more susceptible to being Frozen. When the Marked target dies, a Cold nova erupts from their corpse. This nova does not use your Weapon Damage, and instead uses its own base Attack Damage.

When I try to mark (Voltaic or Freezing) the Ice Crystals generated by Frost Wall or Frozen Locus, I can't.

It seems like a bug, because I can attack them like normal enemies. I'm not sure why this interaction wouldn't work.