Use a brush and a tiny amount of dish soap and water.Rinse the feeder thoroughly to remove all traces of the soap. If there is a soap residue left behind it could alter the taste of the nectar and repel the hummers. Another method is to use a water and vinegar solution, about a 4 to 1 ratio of water to vinegar. Again, rinse the solution well and leave no vinegar residue.If you see mold present in/on your feeders, you need to change your nectar sooner with more frequent cleanings. (I clean my feeders every time I change the nectar to avoid a mold situation. Some clean their feeders at every other nectar change.)To rid the mold, soak the feeders for 1 to 2 hours in a very mild bleach and water solution. A 10 to 1 ratio should be fine.  The soaking will kill the mold bacteria and prevent further growth but the discoloring will still be present. After soaking, clean the feeders thoroughly with brushes.