The mood created is a love of the natural beauty found in the world. Transcendentalism, a movement strongly associated with Thoreau, possessed several fundamental beliefs. One such notion was the idea that the natural world created its own unique beauty which could only be fully appreciated with the immediate sensation of experience. In the passage, Thoreau is identifying this experience of seeing the morning break on the pond and the natural sensations around him. The description of dew and the "throwing off the nightly clothing of mist" helps to illuminate this experience. Highly emotional, Thoreau uses his own physical and mental experience of sensation and sensory images to help grasp the beauty of nature, as opposed to a scientific and detached analysis.
Let's put the passage here for reference:
For the first week, whenever I looked out on the pond it impressed me like a tarn high up on the side lakes, and, as the sun arose, I saw it throwing off its nightly clothing of mist, and here and there, by degrees, its soft ripples or its smooth reflecting surface was revealed, while the mists, like ghosts, were stealthily withdrawing in every direction into the woods, as at the breaking up of some nocturnal conventicler. The very dew seemed to hang upon the trees later into the day than usual, as on the sides of the mountains.
In this passage, Thoreau is simply describing the way the pond looks to him during the first week that he lives there. To me, the words that Thoreau uses here paint a very peaceful and serene picture of the pond. It sets a very calm and relaxed and happy mood.
To me, the words and phrases that do this are ones like "soft ripples" and "smooth reflecting surface." These phrases give the reader a very peaceful feeling. The same effect is achieved by the metaphor of the pond throwing off its night clothes. These are all very safe and peaceful images.