Rinse the dried shiitake mushrooms under running water a few times to rinse away any dirts and impurities. Soak mushrooms with clean water for about 20 minutes. Then trim stems with a pair of scissors.
Return the mushrooms into the large bowl and pour enough clean water to cover the mushrooms. Wrap with cling wrap and soak it overnight at room temperature.
Put the black moss in a medium size bowl. Add water enough to cover the black moss and add few drops of cooking oil and soak for about 5 minutes. This will loosen the black moss and allows the dirt and particles sink to the bottom. Strain the black moss through a sieve and rinse with water and drain well, set aside.
Lightly squeeze soaked mushrooms and save the mushroom water. Heat up wok with cooking oil and stir fry mushrooms for a few minutes until aromatic at medium heat. Pass mushroom water through a sieve into the mushrooms. Bring the mushroom water to boil and add rock sugar and salt to taste.
Then pour all the mushrooms together with the liquid into slow cooker. (Add more water enough to cover the mushrooms). Turn on heat and let it the mushrooms braised in the slow cooker for about 2-3 hours. Add black moss into the braised mushrooms and simmer for another 20 minutes before serving.
Add a little more water into the braised mushroom if water dries up. Adjust seasoning before serving if necessary or add some oyster sauce if you like.