It's beneficial. This is from mongabay.com
"The scientists showed the importance of trapeziid crabs by gently removing crabs from sections of the two species of branching corals on a coastal reef. This resulted in 50 to 80 percent of those corals dying in less than a month. By contrast, all corals with crabs survived. The nature of this common symbiotic relationship had not been recognized until this study. For surviving corals that lacked crabs, growth was slower, tissue bleaching was greater, and sediment load was higher. Laboratory experiments revealed that corals with crabs not only shed substantially more of the sediments deposited on coral surfaces, but also that crabs were most effective at removing grain sizes that were most damaging to coral tissues. These were the largest grains studied, those measuring two to four millimeters in width."
