A green plus (+) sign around a Superstar indicates that a Gear Piece is available for that Superstar.
To equip the Gear Piece:
Tap the green + sign.
Tap Equip.
If the Gear Piece hasn't been crafted yet, you'll need to tap Craft first. Some Gear Pieces may also require WWE Coins to complete the crafting process.
Equipping Gear Pieces increases your Superstar's stats and helps improve their overall Power.