I never know what to call it but I've seen "hidden" for the mutation that combines with lesser to make platy and I think that is as good as any because it describes the mutation. Other names I’ve seen are platy sib or now mystery. Apparently they look like normals unless combined with another mutation in this complex. Even the homozygous hidden seems to be a normal.
But the fact that platy X normal never produced a platy and that the butter platy was produced without inbreeding tends to indicate that hidden is another different mutation of the same gene as lesser, butter, mojave, phantom, mystic, special, Vin Russo, and mocha (and I'm probably forgetting some).
Many of the combos of different mutations of this gene are white (like lesserphantom = karma) but some aren't like mojavemystic = mystic potion. So the lesserhidden = platy combo appears to be another non white combo in this complex like mojavespecial = crystal.
The important thing to remember about different mutations of the same gene (alleles) is that each parent can only give one copy of each gene so a combo like platy can’t give both lesser and hidden to the same offspring. This prevents platy X normal from producing platy and crystal X normal from producing crystal but these breedings also don’t produce any true normals because they all get one or the other mutant version of the gene from the combo parent.