Your black moor is a much slower swimmer compared to the other fish so it cannot compete for food. This is one of the reasons we recommend keeping "extra" fancy goldfish in species only tanks and even fancy goldfish should not be kept with long-bodied goldfish which are much faster swimmers.
What size tank are they in? Maybe you could start feeding the long-bodied fish on one end and then feed the black moor on the other end. It will take a few attempts for the moor to learn to go to the other end at feeding time but it will learn... but it would still be much better to have the fancies in a separate tank or move the long-bodied to a pond where they do best.