I see various critters in my tank on my toadstool...Sometimes small hermits all up in my zoos and such. They are just cleaning. Linkia (True) have all been reef safe by my reading. Some quotes...
"...blue Linckia can grow to 12 inches and feeds mainly on detritus, organic matter, microscopic invertebrates and microalgae. It does very well in a mature reef tank once it's acclimatized with care and patience."
"...The Blue Starfish is probably the most popular and best starfish for introduction into a reef tank. This starfish is unafraid to show itself during the day and will not feast on live rock or clams. It is completely blue in color and very attractive. The Blue Starfish is common in pet stores and relatively inexpensive. They are susceptible to disease and are often found deteriorated in pet stores. If you find a healthy specimen it is a highly recommended addition to the reef tank."
"The Blue Starfish may grow to 12 inches. Omnivore, like to eat filter feed (invert food, brine shrimp, micro-plankton) a few times per week, when open.. The Linckia laevigata is generally peaceful toward other tankmates. Reef-safe. Many consider the Linckia laevigata a low-maintenance specimen. Not venomous. Algae eater. Needs places to hide. Be careful with copper-based medication and extreme nitrate levels. Keep water quality high (SG
1.023 - 1.025, pH 8.1 - 8.4, Temp. 72 - 78Â° F). The Blue Starfish is commonly collected from Fiji."
_Just a few. Does this mean yours hasn't turned to the dark side..no, but if he was grazing on your corals, youd see some very unhappy critters in its path. More than likely just chowing on some algea or just haning out in a place it feels safe or with enough room.