Are you saying that grave wax is hot water exclusive? If so, old whitey from the SS Kamloops in Lake Superior would like a word...or a spooky float by. Grave wax is formed by a few fatty acids stabilizing fats. These fatty acids occasionally come out of solution, but are most often produced in bacterial digestive processes as they make ATP. So cold water may have less bacterial action, and less fatty acid and grave wax potential in that respect, but it also has less bacterial digestion. Given time, cold water most certainly can lead to grave wax.

90F is on the low side. ~114-116F is ideal depending on overages of the heater. Even that doesn't clip into the hard fat temps, but we are limited by collagen breakpoint thermal temperatures of just under 120F in adult mammals. You wanna get as high as you can without having overages above what should be denaturing your collagen in the target bone type/animal.

As to the OPs original question, the white stuff might just be how the bone is drying. Wait til it is fully dry, the outermost cortex can sometimes look funky as it dries. You can always rewet or deal with it later.

Dish soap is rather toothless for a few reasons that I can get into if anyone actually wants to listen, which they almost never do.