Why is there an e in leucobryum?
heard as lucobryum
spelled leucobryum
Because the Greek word is leukos, and eu is one unit of spelling. Dropping the e gives lucobryum, which is not the name.
What the ear did
English speakers hear one vowel at the front of the word and write one letter for it. The e is doing historical work rather than audible work: it marks the word as Greek and keeps it in the same family as leucocyte, a white blood cell, and leucism, the condition that leaves an animal pale. All of them come from leukos, white, and all of them keep the eu. In this genus the whiteness is real and visible. The leaves are built largely of big empty cells that hold water and fill with air as the cushion dries, which turns a green plant grey-white in a few days of sun. The name records what the moss looks like on a bank in August, and the spelling records the language it was named in.
L-E-U-C-O-B-R-Y-U-M
The full entry for leucobryum covers the name itself.