What is the "grated yam" in okonomiyaki?
It's "a slimy potato" - http://en.m.wikipedia.org/wiki/Dioscorea_opposita.  It might be available in Asian grocery stores.  

You could probably get by without it.  Okonomiyaki translates literally as "cook as you like it".  It's a savory pancake-like batter, to which is added whatever you want; often whatever you happen to have lying around the kitchen.  Much like an omelette, okonomiyaki is a pretty free form thing.