It's hard to tell from the pic, but usual spawning behavior for trochus is what you described. They (two or more snails) hang near the water surface and release a milky white cloud. If they were successful, then you'll see little ones soon enough. Free CUC!
My turbos are doing this right now. First the big one started, then three or four more joined in. Been going on for over an hour. The fish are going crazy gobbling the eggs up. I hope some of the snails survive. I put some into my sump and QT tank.