It looks like a relatively large Vermetid snail to me. I'm not sure on the species, but that tube is very unique (if you could get a clear, closer up pic, that may help with the ID, given the unique tube). Vermetids are generally considered pests, as they reportedly sometimes irritate corals. However, it likely depends on the coral and the vermetid species, as some people have pics of their corals growing over the vermetids and doing just fine. I've never heard of rapid spreading with the larger species, but they seem to be much rarer than the smaller ones, and people tend to remove them as soon as they see them - so they might just not be in the tanks long enough to spread like the smaller ones.
Most people would say to take it out, though some consider them harmless, and I know at least one person that considers them somewhat beneficial.
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