There is a similar thread here discussing using the bees to clean the Flow Frames. Bees cleaning flow frames after removing super
If it is really bad, then a pressure washer can work, making sure the Flow Frame is fully secured and on a flat surface. Start from a good distance and bring the nozzle closer, slightly angled until you notice the mould coming away, dont get too close as there is a risk of the Flow Frame being blown apart or damaging the plastic components.
