The old miner, also called the "pocket-miner," is addressing the canyon itself as "Mr. Pocket." This pocket section of the canyon is pristine and the miner believes it will provide him with a nice supply of gold. He even designs his system of panning in a V-shape, also resembling a pocket. As he is looking for gold in order to make money, the notion of a pocket as a place where one puts money (wallet, wallet in the pants pocket, etc.) is probably a pun London was employing on purpose.
Pockets are also hidden. We might be able to see the pocket's opening but usually not in the pockets themselves. The miner discovers this hidden pocket and, being alone except for his horses, begins talking to this pocket of the canyon as if it were a person or as if he were addressing the spirit of this hidden place (where the gold is hiding). However, he is only in it for gold and he does not really connect with the spirit of the place as it is described before he intrudes on its peace and stillness:
The hum of the bees and the whisper of the stream were a drifting of sound. And the drifting sound and drifting color seemed to weave together in the making of a delicate and intangible fabric which was the spirit of the place.
The miner has a different concept of the spirit of the place. He believes the spirit has to do with gold, not peace and tranquility. When he addresses Mr. Pocket, he is addressing the gold itself, or the personified landscape containing the gold. A more ecological or nature-loving hiker might have addressed the canyon with names such as Mr. Tranquility or Mr. Whisper. The miner had money in his pocket on his mind.