(OOC: Shriekers are set off by movement and light, but they tend to focus more on the movement of actual living creatures than inanimate objects, particularly other plants and fungi (as it says they like to work with Violet Fungi in the MM, and I would assume they don't trigger every time the Violet Fungus twitches). Thus, it would be likely that arrows covered in moss would not trigger the Shriekers)
(OOC: Okay, then in that case, I would suggest against a Rapid Shot, since that's likely to do enough damage to kill it outright. Of course, you could also wash off the moss residue from Rillian or something )