My trochus did this, and I assumed it was a breeding thing. Like your video, my snail perched itself on a high spot, with high flow. That's why I noticed it to start with... it was not one it's "normal" spots to be in. Just as I went over to check it out, it did just like your video... released a fairly large white stream of stuff. Did this five times, then moved on. I've never seen any of the snails do it again.
Interestingly, last month - a couple months after I noticed the "white cloud" - I noticed baby trochus snails in my tank, all about the same size. There are three that I've seen so far. Not sure if there are more.