Why does my cauliflower puree taste mustardy after two days?
The flavor / odor notes you're experiencing are probably sulfuric compounds. Cruciferous vegetables like broccoli and cauliflower contain sulfiric compounds that are released on cooking. That's why the stir-fried crisp-tender broccoli tastes so different from the limp overcooked olive green florets. Similar to how spicy salsa fresca grows ever spicier after a couple days in the fridge, your puree is slowly continuing that "cooking" process. 

It's still okay to eat, but you'll be less aware of the flavor change if you use it in a more highly spiced dish once it's a couple days old.