Mishika can be a japanese name - i.e. it is possible to spell it with japanese characters - but it is not really being used. If you type Mishika into google first thing you get is that it's being used as a transcription for the word "mythica", but not as a name (apart from a dog's name).
Mishika doesn't have any fixed meaning in Japanese either.
A name that is much more frequent would be "Michika", but the pronunciation is different and it doesn't have a fixed meaning either. Else
I can only think of Misa or Misha
which are close.
But I suppose Mishika has a different origin, maybe a made-up name.