While separating symbionts from live corals using formalin or alcohol, excess mucus is secreted by the Coral Polyp or the colony, which in turn makes difficult to collect the symbionts & also reduces the collection / separation of symbionts.
Try using a solution of magnesium chloride to anesthetize the animals first, and then collect the samples. Concentration and time will vary based on species/size, so you may need to play around with this some... a certain amount of mucus is inevitable though. Good luck!