DO NOT dump DE in until you figure what you really have. A sand filter is nothing but a container almost full of filter sand. Incoming water is sprayed on the top and there is a mechanism to drain water from below the sand for the outlet. The valve assembly can be either top or side mounted but they do the same. Control the path of water flow between inlet from the pump, outlet to the pool, and waste. Have no idea what the other stuff is but something could have gunked up the top layer of sand. If you dig below the surface a couple inches see what you find. Should be just dirty sand. A DE filter is constructed totally differently. Please dig up any model numbers and any info that you can find to identify the equipment. Some info on the history of your pool would be useful too. You said it was one year old. Did you acquire it with a house? Install it yourself or have someone install it?
Al