Can't see the picture very well… Impossible to diagnose, but...
Bad!
If it doesn't come off easily by syphoning you could try a Hydrogen peroxide dip.
15% of Hydrogen Peroxide and 75% tank water for 1 or 2 minutes.
Get the zoas and rinse them off in another container with tank water before returning to the tank.
Make sure you solve the algae problem by controlling the nutrients in the system.
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