Grassy Terrain creates a low-lying grassy field for 5 turns, that restores the HP of grounded Pokémon each turn, by 1⁄16 of their maximum HP. The power of Grass-type moves also increases by 50% while in effect.
Grounded Pokémon are simply those that are not raised. Raised Pokémon are any Flying types or those under the effects of Levitate, Magnet Rise or Telekinesis. Full details in glossary.
so basically raised =/= grounded.
Showdown proof and source.
Since after using Magnet Rise you are no longer a 'grounded' Pokemon, no you will not be healed by Grassy Terrain.
