When asked if whether Visorak venom is a poison or not, most knoweldgable enough on canon would probably say the same thing Greg has, that it isn’t because it is not fatal (1, 2, 3, 4). However, I think I can make a pretty strong case against this, that Visorak venom is in fact a poison.
First, Greg calls it poison several times (5, 6, 7) and Visorak themselves are called “poisonous scourge” (“Visorak.” Encyclopedia Updated, p. 152.). Besides just arguing from semantics, Greg elsewhere defines poison as causing illness OR death (8, 9, 10). Despite Greg saying Visorak venom is not poison because it doesn’t kill, he defines poison as also just causing illness, not necessarily death.
One could also argue that Visorak venom is actually a mutagen (11). However, mutation is only physical (12), but Visorak venom affects its target psychologically too (13, 14, 15, 16), just as Lerahk’s poison affected Tahu (17).
The objection could be raised then, that if Visorak venom is a poison, it can be cured simply with a Remove Poison disk, so why didn’t the Toa Hordika use one? There are several possible answers to this. One is that they simply weren’t thinking clearly at the time (18). But even then, why didn’t the Rahaga think of this either, since they were still wise while mutated? Another possibility is that the venom only worked for a short period of time, then its effects were complete, after which point a Remove Poison disk would’ve been useless (19). A third possibility is that while Remove Poison could theoretically cure Visorak venom, we don’t know what level is required to do so. If they needed a higher level disk, say 7 or 8, then it may have been too difficult and time consuming to try to craft one, and it wouldn’t even be guaranteed given the broken state of Metru Nui (and the Great Furnace specifically) at the time. They probably did not have the resources needed to create one.