One l or two?
heard as pollytrichum
spelled polytrichum
One. pollytrichum has a letter too many: polytrichum.
What the ear did
A short o before an l is usually written with a double l in English words: jolly, folly, holly, trolley. That habit is strong enough to reach across into a Greek name where the l belongs to polys and stands alone.
P-O-L-Y-T-R-I-C-H-U-M
The full entry for polytrichum covers the name itself.