This is my take on the crystal hearts disease, the same disease that @SHADOWSTEIN 's Mortis suffers from.
My headcanon is that it is a congenital disorder caused by several frog and jellyfish gene sequences being active when they normally aren’t in a healthy fluffy. The disorder causes the fluffy’s skin and muscle tissue to form gelatin like and near transparent. The skin is even weaker than a fluffy’s normal skin and very prone to dehydration. The muscles and even the organs are far weaker than in a normal fluffy. Because of all this, fluffies with the disorder are usually stillborn or die within the first few days of life. It is almost unheard of for them to make it to adulthood even with medical aid.
Thanks. I was actually going for a more realistic take on the disease since you can’t really have the completely invisible look of complex organic tissue unless it pretty much has the consistency of jelly, which would basically just slough off under its own weight. Admittedly, that would be a very interesting demise for a fluffy. Glass frogs are about as close as you can get to being transparent, at least as far as a complex organism.
Turn bones into Iron (like actual industrial-grade iron) and fur into a gasoline or oil-like composition both pulling double duty of being cared for by workers and giving a grisly fiery death resembling that of a Wicker Man
Give bones the durability of tree branches and rearranging the fur into a state where it could be comparable to barbed wiring, which poses a risk towards itself and others (especially females)
But that’s the Wicker Heart and Barbed Fur diseases for you.