I have never heard of sexy shrimp eating a LPS, or any coral for that matter. As the site that you link to mentions, some LPS may get annoyed by the attention of a sexy shrimp (i.e. the sexy shrimp walking over it) and, like any shrimp, I suppose the sexy may steal food from coral from time to time, but actually feed on the coral? Are you certain you did not misinterpert what was happening?