cladenia or cladonia, and does the vowel matter?
heard as cladenia
spelled cladonia
It matters for finding the word again. Cladonia is the spelling.
What the ear did
Search engines are forgiving with one letter and Latin indexes are not. The o is the join between klados and the Latin ending, the same join you can see in Cladophora and cladogram.
C-L-A-D-O-N-I-A
The full entry for cladonia covers the name itself.