polietrishum: two guesses at once
heard as polietrishum
spelled polytrichum
The y has been written ie and the ch has been written sh. Both are wrong here. It is polytrichum.
What the ear did
English gives ch three different jobs, and only one of them is right in this word. Nothing in the sound rules out sh, since ch does say sh in machine and chef. The tiebreaker is the source language, and Greek chi always lands on k.
P-O-L-Y-T-R-I-C-H-U-M
The full entry for polytrichum covers the name itself.