Commercially prepared Mostarda gets its kick from mustard oil, which is clear and thus doesn't cloud the syrup. It is, however, difficult to find, because it's a powerful irritant. Your best bet will be to visit a herbalist, who will likely have to order it for you.
Drain the fruit into a pot again, and remove about a shot glass's worth of syrup from the pot. Set the pot to heating, and in the meantime carefully add 10-15 drops of mustard oil to the syrup in the shot glass.
If you spill a drop or get some on your skin, wash it off immediately with warm soapy water.