While this is a fair effect in Limited., for a 3WW mana investment, you’re better off flashing in Dictate of Heliod for a permanent +2/+2 boost for all of your creatures. So why you would ever play the "Master of Pearls" outside of a Limited environment?
If Master of Pearls is manifested by the Mastery or Whisperwood Elemental, you get a chance to flip it for 1W and gain its effect, instead of having to pay 3WW. Not having to play it first face-down for 3 colorless really improves this creature's effect. Of course, if you end up with Master of Pearls in hand, you're still going to have to pay its full cost.
Even with the possibility of getting lots of value out of Master of Pearls if it gets manifested, the potential drawback of drawing into it may not be worth including it. The potential of creating a Manifested army and swinging for the fences with a +2/+2 boost is pretty cool, though. While Master of Pearls may never be more than a bulk rare, at least there is potential now for it to potentially help a Manifest deck land a crippling blow out of nowhere.