The picture looks kind of like a vermetid snail assuming it has a hard shell and tube. Not terribly bad as long as the population doesn't explode. If they get out of hand they can be unsightly and can grow in places you don't want them, but as pests go, not the worst either.
More info here.
http://reefkeeping.com/issues/2005-01/rs/index.php
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I retired and got a fixed income but it's broke.
Ed
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50 gallon FOWLR, 10 gallon sump.
130 gallon reef, 20 gallon sump, 10 gallon refugium.
10 gallon quarantine.
60 gallon winter tank for pond fish.
300 gallon pond with waterfall.
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