The model is white because its textures are missing or not loading. I couldn't even open it in RMS. I tried testing in SD and HD mode in editor but to no avail. Usually mdx files have their texture files with them, what is the source of the model if I may ask ?
Edit: I managed to open it, the embedded texture data inside the MDX is broken or incompatible. That’s why Retera won't open it (at least for me) , War3Mdl editor shows it black, and the game shows it white. You’ll need to extract or replace the texture, or rebuild the model with a proper .blp instead of the embedded one. Btw is this model from Reforged? I sense this is a HD/Reforged texture or format being used in an SD pipeline.