The Teal Mask:
-No effect.
The Wellspring Mask:
-Puts Ogerpon in her Wellspring Mask form as a duel Grass/Water type
-Increases the power of moves by 20%
-Restricts Terastillization to Water
-When Terastillized changes ability to Embody Aspect and raises special defense
The Hearthflame Mask:
-Puts Ogerpon in her Hearthflame Mask form as a duel Grass/Fire type
-Increases the power of moves by 20%
-Restricts Terastillization to Fire
-When Terastillized changes ability to Embody Aspect and raises attack
The Cornerstone Mask:
-Puts Ogerpon in her Cornerstone Mask form as a duel Grass/Rock type
-Increases the power of moves by 20%
-Restricts Terastillization to Rock
-When Terastillized changes ability to Embody Aspect and raises defense
Source: Experience and Bulbapedia