If it was me .. I would
A .. get rid of the coral rubble in the sump
B .. Get a RODI filter ( if you don't have one ? ) I read in your tank journal that your using asw for water changes , With the water changes plus top ups .. It would be hard to export that amount of phosphate that would come from the tap water , It would only have to be a low amount in the tap water and it will build up over time
c .. Manually remove as much as you can ...If you leave it there , When it dies it might just release nutrients back into the water again
d... Keep your chin up , Most of us have seen this in one of our tanks at some stage