When a creature infected with the mold dies, it calcifies and sometimes crystalises. In creatures with more unique mutations from the mold, it crystalises into, well, crystals but in common strains it simply calcifies.
Of the four Lords, they all leave behind different remains: Lady Dimitrescu leaves a crystallised version of most of her body from the mid-thighs up with a large hole in her chest area and missing her lower arms; Donna Beneviento leaves a large amount of calcified substance and her doll Angie; Moreau leaves a cluster of crystallised tissue with a strong resemblance to his back; and Heisenberg leaves a chunk of crystal mingled with metal parts.
I think that these were the parts of their bodies where the mold was at its highest concentration. The lycan’s heaviest mutations were on their faces with the fangs etc, so we can assume that’s why only those parts crystallise.
Lady D’s regenerative powers meant that the mold was widespread throughout her body, and the hole can be assumed to be where her Cadou was. Her dragon form lacked arms and her legs became the, y’know, dragon part which as we see in game completely calcified. Though you do have to think about how impressive a Whole Crystal Dimitrescu including the dragon part would be.
Donna’s infection made her secrete a pheromone to control local hallucinogenic plants infested with the mold, so we can assume the part of her that would crystallise was either her sweat or her sweat glands. We never see liquid successfully crystallise ingame, so we can assume that’s why she completely calcified. The only remaining part of her cadou was in Angie which we retrieve and can probably assume is still alive. She is the closest thing we have to a Crystal Beneviento, and she’s most likely not quite dead unless she crystalised when the main cadou implanted into Donna died.
Moreau’s back is horrifically mutated, as we can directly see ingame. His crystallised remains bear a strong resemblance to this, so we can assume that this heavily-mutated part of him was what crystallised.
Heisenberg died after, according to the re wiki, “stretching his body apart and spreading his biomass through a large mechanical apparatus he assembled in and around his flesh, becoming a cybernetic mutant.” Admittedly the dying part was because of our dear Mr Winters, but anyway Heisenberg did most likely mingle the electric organs his body made after infection with the Cadou parasite with the metal pieces. As such when the organs crystalised they were mingled with metal pieces, forming the Crystal Heisenberg.
TL:DR I think that upon the death of its host mold crystalises the part it mutated the most, and regular-ish human tissue simply calcifies.