Filters, especially internals, do not remove the fish waste from the bottom of the tank. It is the fishkeeper's job to remove that during a water change. If there are live plants growing in the gravel, they will use it as fertiliser as it breaks down.
The Fluval U series has a central box full of biomax, a sponge (foam) on each side of that, and on the outside two polycarbon pads.
The biomax and sponges just need washing in the water you take out during a water change. The polycarbon pads should be changed regularly. The carbon bit gets full and stops working, and poly bit will get clogged easily. Since you don't need to use carbon routinely, buy a sheet of filter wool/filter floss and cut it to the same shape as a polycarbon pad. Filter wool/floss doesn't wash well, it goes holey and shapeless so just put a new piece in whenever one gets clogged with brown goo.
Green water could be an algal bloom - microscopic algae floating in the water. You need to find out why it is occurring, and make changes to stop it. Plants, including algae, need light, fertiliser and carbon dioxide to grow. If one of these is out of balance with the other two, that will cause algae to grow. For example, how long are the lights on for?