The Carvicts are not sapient. They may be clever, but to say that such creatures could hold debate would be ridiculous. Nonetheless, that is precisely what appears to be happening. Large numbers of the crustaceans cluster together, scuttling between and atop each other, as space opens around pairs or singles. Several hours or even days later, they would disperse, returning to their individual colonies. Really, it was a mating ritual, and they were breeding locations, but they were also opportunities for socialization, of a sort.
This parasite didn't hurt them, most of the time, but there were multiple solutions to this issue that breeding could and likely would take. The easiest was a behavioral shift, as Carvicts groom each other, cleaning off the spikes, use their hind-claws to scrape off the mold/moss and consume it. It was more beneficial to leave the rest and groom it away again later, as it could also afford some small amount of camouflage, hiding Carvicts from predators more easily.
The second possibility, and one likely to occur with more isolated colonies, was a simple on. While they did not truly excrete toxins to the degree most poisonous creatures did, as usually the growths which stored the poison not only grew only if there was sufficient poison to need or want storing, but also served mainly to break at the tip and fill predators with an assortment of venom that way, there was certainly the possibility of it. And if it did excrete poison in such a manner, then the problem would solve itself; the spikes would be poisonous, so the parasite would be unable to grow there. Of course, these would mean a greater need for toxin consumption, even if it meant that predators would be held off even more effectively.
Which solution would spread to the entirety of the species, if either would, was another question entirely.
[A map would be cool]