I am not a real knowledgable fellow regarding seastars, all I could comment is that your one (Pentagonaster duebeni) was/is probably feeding on the coral polyps. Most seastars are predators or scavengers, so I think it is quite possible that this little red beast got the better of the coral.
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It was one of those places where the forces which govern life seem to come to a focus of preternatural beauty and intensity. I stood in a small grassy field. It was bounded by a dazzling band of coral sand. Crystalline aquamarine shallows grading into turquoise surrounded the islet. Farther out, a broad shallow reef flat fashioned a skirt of many colors terminating in a hemline of sparkling white breakers. Beyond, an endless sea of indigo blue velvet spread out to the horizon. Life was everywhere.