leucubryum: the vowel in the middle
heard as leucubryum
spelled leucobryum
It is o. Leucobryum, from leuco- and bryum.
What the ear did
That vowel is the join between the two Greek words and carries no stress at all, so speech reduces it to a neutral sound that could be written with any vowel. The o is there because Greek compounds use o as their linking vowel, in leucocyte, chromosome and thermometer alike.
L-E-U-C-O-B-R-Y-U-M
The full entry for leucobryum covers the name itself.