Trioxane fuel tablet recrystalization
It's subliming (aided in this case by decomposition) and re-depositing.

Substances that turn directly from solid to vapor (e.g. naphthalene, water ice) are said to sublime, rather than evaporate. The inverse, changing from vapor to solid, is termed deposition.

When water vapor deposits on very cold surfaces, such as "dry ice", it grows into amazing dendritic crystals: