Q:

Is Tribal a permanent type (for Muldrotha)? 

I am building a Commander deck with Muldrotha, the Gravetide as the Commander.  During my deckbuilding, I remembered there is such a thing as a "Tribal" card type.  Is Tribal one of the supported card types that can be cast from the graveyard using Muldrotha's ability? 

A:

Tribal is a card type but not a permanent type.   So Tribal counts for cards like Tarmogoyf, but not Muldrotha, the Gravetide. 
From the comprehensive rules:

110.4.  There are five permanent types: artifact, creature, enchantment, land, and planeswalker. 
300.1.  The card types are artifact, conspiracy, creature, enchantment, instant, land, phenomenon, plane, planeswalker, scheme, sorcery, tribal, and vanguard.