More than likely, the fluid you are seeing is either water (if the preening occurs after bathing as the water is coming off the feathers) or saliva.
The swan rubs its head and neck at the top of the tail on the uropygial oil ( preen) gland. This yellowish to clear oily substance is then transferred along the swan’s body to make the feathers repel water and keep the feathers healthy. There is also some research that states the oily substance may contain antibodies to keep the feathers healthy. Other research states that the oil increases the swan’s ability to detect ultraviolet rays. This may help the swan identify other swans from a distance.
The fluid you are seeing could also be saliva as the swan transfers the oil across its body. The oil may be distasteful and the swan produces saliva as a reaction to the oil or uses the saliva to help move the oil more readily across the feathers. In any case, as long as the swan is eating, pooping and preening, these are indicators that the swan is healthy and acting normally. We hope this information is beneficial. Enjoy the swans! The Regal Swan