Yep as Carl said, laundry detergent, loaded with phosphates! I hope a teaspoon of that didn't make it back into the pool.
It's always a good idea to fill the filter half full of water before commencing a fill. It not only reduces the amount of dust you may breath in but it slows down the impact of the sand on the laterals. Personally I would add course gravel to cover the laterals before the sand goes in.

Backwash the new sand and then rinse....before filtering