If we look at the Japanese word, there is no "Fujiyama" at all. I have a hunch that this was a mistake of a translator who transliterated 富士山 wrongly due to the kanji "yama" reading. Yet if it's a mistake, why was it so widely spreaded? Wikipedia refers to "Fujiyama" as a disambiguation to "Mount Fuji" and I even saw a Japanese book with the title "FUJIYAMA". Was it really someone's mistake or Japanese has some variation about this particular mountain?
For example, here in Russia many people consider "Fujiyama" normal name where those who study Japanese consider it as a terrible inaccuracy of a translator. Where is the truth?