sounds like a type of shield limpet or chiton, more than likely harmless (reefwise) have a little google.yes it moves like a slug.. its white underneath and has a mouth underneath and u can c it actuali licking the glass.. then on top its rounded.. its a greeny colour with like light blue lines and has tentical things all over its top.. and wen theres disturbance in the water its back opens up like a lady bug opens its wings and its white with 2brown lines on either side.. Sorry about the pic.
I know them as shield limpets, but we do live on different continentsIf he says it looks like the back opens like a ladybird opens its wings, could it not be a small/baby cowry of some sorts? They got that manle that they pull over their shell from the sides. Let's hear what the other guys think