I have seen this thing in my tanks for months now and know its a cool thing to have, normally I will see it around when the main tank lights are out but today I see it having a "release" lol
Like Trochus snails, stomatella snails braodcast there sperm and eggs into the water column. Probably difficult to differentiate between what’s being released but hopefully you have one of the other sex in there. As you know being nocturnal and almost camouflaged on the rock, there is a good chance there are more than one.