Anemones produce a very large amount of mucas and are pretty dirty creatures when it comes to organics. What you are seeing is the algae taking advantage of the organic output of the anemone.
As your tank matures and your biodiversity really ramps up it is very likely that this will simply go away on it's own. You can manually remove it, but make sure you stir the sand in that area to encourage more biodiversity from the oxygen that you will provide the sand by stirring it. Perhaps a nassarius will help with stirring the sand also.
Either way though it would be considered unsightly it is nothing to be concerned about.