I am working on a mod in 1.9 and I want to make it so the entity cannot move on any block but snow or snow layer does any one know what I need to get this working

Custom AI, specifically a custom Navigator (might be named something slightly different - I don't recall exactly).

 

Alternatively, you could first attempt overriding #getBlockPathWeight in your entity class and return 0 for any non-snow / non-snow-layer blocks. That should prevent your entity from pathing onto non-snow covered blocks.