And what you're missing is that this is too damn close to mutation - it is no fucking accident that all of the spells Empire Wizards use steer well damn clear of permanent physical changes to the bodies of others' bar healing, because the one thing every person in the Empire fears more than death is mutation. This is nothing less than a culture-wide trauma trigger, as there is not a single person in this entire nation who didn't grow up fearing the touch of Morrslieb. Who did not desperately pray to be spared the Chaos God's getting a grip on their bodies. Who did not take part in the ostracisation and exile-if-not-communal-murder of a mutant. Literally no amount of tactical usefulness is worth even going near that red button.
I think you are way, way overthinking this. It is going to look like silver liquid that turns red when it touches flesh scouring said flesh off the target in part on in whole. Mutation do not just alter the consistency of flesh, that is an incidental quality of some Chaos Spawn. Mutations make mutants, living ones.