I've seen that sort of scum most years at various places in the passage.
It could be sewage from septic, but if that is the case MBRC will be contacting all septic owners to have them serviced....my brother's place at Burpengary must get his serviced every year and maintain records.
My gut feeling it this might be a algae bloom at this time of the year, all the old growth dying off on the huge amounts of weed beds in the passage.
I have seen this in all of the creeks that feed the passage at times, coochin, hussey, elimbah and ningi. Also seen it in Tripcony Bight too.
I saw a big dose of it in the Bongaree Canals about 8 years ago....stagnant water though.
Break out the poppers for tings if the prawns are skipping.