hipogymnia and the hippo problem
heard as hipogymnia
spelled hypogymnia
Hypogymnia, with y in the first syllable.
What the ear did
The i spelling would make it a different prefix altogether. Hippo- is Greek for horse, hypo- is Greek for under, and only the y version has anything to do with a lichen's bare underside.
H-Y-P-O-G-Y-M-N-I-A
The full entry for hypogymnia covers the name itself.