Those chunks look like what I got in my pump basket after my plaster pool had been chipped out prior to my Pebble Sheen re-plaster. After about a week or so after the replaster I started to get them. I didn't have any borax in the pool. I figure it must have been plaster dust/bits clumping together. I did also have a sock of CYA in the skimmer the week after the pool had been filled prior to the chunks showing up??