[Columbine]

I'm not an expert, but I think that Mochi-ko is the middle grade of confectionary flour, and as opposed to ordinary rice flour is made from a different kind of rice altogether. It's finer and sweeter as well as being more glutinous. Shiratama-ko is even more so, and is considered the best. If that is what the recipe suggests, that would be the best flour to use and you should expect a different consistency if you use another flour.

It might depend on what flour you DO have. If it's a short-grain rice flour, it should be ok, just not the best quality, but if it's long-grain rice flour (a lot of korean brands are this, apparently) it probably won't work as it won't be anywhere near glutinous enough.