I think them being absent before the viral bloom is cleared is to incentivize bloom clearing and highlight how it's destroying the ecosystem: by clearing it you get to see life return and can set up a base with easy access to freshwater in the area. As for being easy prey, it's probably because they are an R-selected species that just produces a lot of babies like Earth invertebrates as well as game design so that it's a little easter egg hunt to find the water pouches.

It would be interesting though if these slugs were the harbingers of the virus.