cladonnia and the doubled n
heard as cladonnia
spelled cladonia
The n is single: Cladonia.
What the ear did
The stress lands on the syllable before it, and a stressed vowel in English usually has two consonants behind it. Latin does not work that way, and the ending here is the plain -onia found across the whole plant kingdom.
C-L-A-D-O-N-I-A
The full entry for cladonia covers the name itself.