Trochus snails? If so the sperm will look like "smoke". If you have a several it can cloud your tank. First video is of sperm release, the second shows the eggs suspended in the water column.
When one of my largest turbo snails started doing this, within minutes all of them joined in. The water was a milky mess and the fish went crazy eating it up.
I was hoping for new snails, but that was mid-November and I haven't seen any. Perhaps the fish got them all or they just don't survive in captivity.