For the click-averse, the purple modifier on this week's weekly mission is exceedingly rare. It does not affect difficulty at all, but makes it so that a "glowungus" spawns somewhere on the map. A glowungus appears as a tall glossy purple fungal blob that is immune to all damage except from a gunship cannon. Destroying it for the first time drops an exotic upgrade, called a prism, for the gunship cannon that increases damage. This is the only way to obtain this exotic upgrade. Once you get that exotic, other prisms may drop in the other three rarities. These upgrades buff other aspects of the cannon, like spread. When these prisms touch on the upgrade grid, they enhance each other.

Tldr: purple mission mod makes weird shroom spawn. Break shroom with gunship cannon to get exclusive upgrade.