Sorry if I'm overlooking a rule somewhere, but encounter the monster or no?
No. The flavor text is just for fun. By placing the silver monster face up in the region, you have followed the "rules" text on the card, which is the real effect of the card.
Note: if the card had been drawn during the action portion of your turn (for example, because you had a FF marker on one of your action choices), and you ended your turn in that region, that silver monster would then be an end-of-turn obstacle to encounter.