Lobster mushroom is not, strictly speaking, a mushroom proper, but a parasite, Hypomyces lactifluorum, and the mushroom it grows on. The mushroom of choice is usually Lactarius piperatus or Russula brevipes. An infected mushroom turns a reddish-orange while its flesh remains white, the end result bearing a strong resemblance to lobster meat. "Lobster mushroom" thus refers collectively to the parasite and the host mushroom. You'd do well to not let the symbiosis gross you out, for lobster mushrooms are edible and delicious, with an unexpected shellfish note and a dense, meaty texture.