You may want to check the condition of the spider gasket in the multiport first.
To check if sand continues to accumulate, after the pump is on, brush the area where the sand settles and check again about 10-15 minutes later.
If you do wind up having to open the filter, plan ahead and get some PVC unions to put in line near the multiport valve so you don't have to cut and glue the lines if you need to get into the filter in the future.