Frozen Legion cannot be exerted

Submitting this as a bug report since it doesn't explictly say on the gem that it cannot be exerted (unless I am blind..)

My guess would be that the Frozen Sweep attack is what needs to be exerted and the limitation is that only the statues can use that ability?

Confirmation would be appreciated

That would be correct. The gem itself can't be exerted since it's a spell, which is why you see the spell echo part. This is way over simplified, but spell echo is basically the spell version of Exerted attacks.

Edit: Additionally, since the skill repeats, repeated skills can not be exerted.