That one's been exhibited several times and I'm pretty sure the general understanding is that it's a static reproduction made using some of the original molds and potentially some screen-used costume elements.
Looking at the way it's aged over time I'd guess that it's a slip-cast latex piece (more durable than foam latex but still prone to drying/discoloring over time).
I've seen several of the original screen-used Yoda faces up close, and they were indeed foam latex (and most are in a bit of a sorry state now, I'm afraid!)
Tom