I can get it down to one of two species, either Gonodactylaceus glabrous or G. graphurus. They are very similar species that are found in shore in shallow water. Both get to 8-9 cm. G. glabrous is common in Indonesia; G. graphurus is common in Australia. To distinguish them, I would have to see a couple of other characters, particularly the median dorsal surface of the 6th (last) abdomenal segment, a dorsal view of the rostrum and a close-up side view of the addomen. These to species are very closely related and are probably sibling species.
Roy