Throwing around MSG with black pepper as though white pepper isn't already the perfect pairing.
But for serious, with its earthy flavor, you might find that it compliments the mushrooms a little more. It's also already husked, so taking out some of that peppery bite helps the other flavors in the dish shine just a little more.
More to OP's benefit, the way that white pepper is processed sometimes makes it a better alternative for sensitive tummies.