Hong Kong stocks started the week on a muted note Monday, opening marginally lower after clocking up five days of healthy gains.The Hang Seng Index edged down 5.87 points to 21,653.38.And the benchmark Shanghai Composite Index retreated 0.22 percent, or 6.66 points, to 3,047.64, while the Shenzhen Composite Index, which tracks stocks on China’s second exchange, slipped 0.23 percent, or 4.71 points, to 2,034.02.