This is not flukes but rather mucus plugs AKA mucus cones that blue tangs are very prone to getting from likely ich mixed within. You will need to place fish in Quarantine tank and treat with Coppersafe or Copper Power at therapeutic level 2.25-2.5 For a FULL 30 days (do not interrupt this 30 day period) monitored by a reliable Copper Test kit such as Hanna Brand- No API brand. Also monitor Ammonia levels while in quarantine with a reliable test kit and add aeration during treatment using an air stone.
The display tank should be kept fishless (FALLOW) for 6-8 weeks to assure ant existing parasites go through their life cycle without a host fish and die off
A quarantine tank can be as simple as a starter kit from Walmart which most of the needed essentials.