Dear Andy;-
     Spending summers at Folly Beach near Charleston, S.C., I have
seen D. variabilis by the thousands on their tidal migrations. (They
come in many colors and variations (thus their name) but always the
same shape and about a length of 15mm). I have never noted that they
are coated with algae. I have, however found halves of dead D. v with
a few strings of algae attached at the ligament. I would venture to
guess that their being carried up and down the beach so often would
provide a sort of sand-blasting that might loosen algae from its hold.
 I had surmised (without proof) that the algae had only got hold dead
D. v that no longer migrated.
      Bad guess, huh?
         Art