Toast the almonds in a dry skillet over medium heat, stirring often, until fragrant and lightly golden, 3β5 minutes. Transfer to a plate to cool.
Segment the grapefruit over a bowl to catch the juice. Cut off the ends, remove the peel and white pith, then cut between the membranes to release segments. Squeeze the remaining membranes to collect juice; measure out 2 tbsp juice for the dressing and reserve the segments.
In a small bowl, whisk together the olive oil, 2 tbsp reserved grapefruit juice, lime juice, maple syrup, and half of the salt and pepper until emulsified.
In a large bowl, combine the baby arugula, shallot, and half of the mint. Drizzle with about half of the dressing and toss gently to coat.
Halve, pit, and slice the avocado. Divide the dressed greens between two plates. Arrange the grapefruit segments and avocado over the greens.
Spoon the remaining dressing over the grapefruit and avocado. Sprinkle with the toasted almonds and remaining mint, and finish with the remaining salt and pepper. Serve immediately.