It sounds like mating behaviour thats going on and if the eggs are sucsessfuly fertilised you could have hundreds of fry but only a few may survive and make it through to adulthood.
The white spot that you appeared to see could have been breeding tubercals (check Spelling)
What I do to control unwanted weed growth in my gdn pond is wind it around a stick as soon as I see it (little & often) or an old loo brush can handy for the job. All the Best Tbird+



