Thanks for confirming it's sand. I would order the spider gasket for the multiport and open it up. It's a pretty easy and cheap fix if that's what it is.
If not, then I think you are looking at removing the sand and checking the laterals as well as the positioning of the center pipe (which can sometimes come unseated)
If not, then I think you are looking at removing the sand and checking the laterals as well as the positioning of the center pipe (which can sometimes come unseated)