It sounds like they are releasing gametes in the water to me. A cool sight in deed. But turning the pumps and filters off is a big mistake. You have to remember that these newborn anemones (if it is a male and female releasing) take up oxygen. And in that dense of a concentration of babies, you can quickly deplete all oxygen and crash the tank. Also, as the oxygen rapidly depletes, the babies begin to die and any excess gametes all breaks down and goes through the Nitrogen Cycle. If something isn't done to filter this out, you will have an ammonia spike that, once again, could crash your system. The bad news about attempt to propogate something like this through sexual reproduction is that the babies are extremely tiny and often food to other filter feeders. In order to attempt to keep as many of them alive, you would need to be using phytoplankton in a designated growout tank. The larvae of the anemones are free floating plankton. I can only speculate that it would take several months before they settle down out of the water column, and many more months after that to be a substantial size to visibly identify them from other tiny critters in the tank.
With all that being said and the extremely low chance a single anemone larvae could make it, you should be considered lucky. It wouldn't surprise me if your chances of winning the lottery is better than raising anemones through sexual reproduction by chance.
