Why do people write marshansha?
heard as marshansha
spelled marchantia
Because they heard a French ch. The plant is Marchantia.
What the ear did
In French, Marchant is said with the sh of machine, so the pronunciation is not wrong so much as un-Latinised. Botanical Latin hardens ch to k in almost every name it touches, and the spelling stays ch either way.
M-A-R-C-H-A-N-T-I-A
The full entry for marchantia covers the name itself.