saltwater ro water bucket with problems

[simple si]

i have just taken out my circulation pump out of my saltwater ro water butt and it is covered in like a brown sludge!!! any ideas? i have been having a problem in my 240 litre tank with small amounts of algae and cyano . my tapwater is runnning thro the usual filters etc and is reading 0 and i have tested with api to see what the readings are before adding any to my sump and the readings look ok except nh3 which is between 0.25ppm and 0.50ppm all other s are fine.
all my corals and star polyps are looking fine as are the fish, but with all this sludge i am unsure wether to clean the bucket out or not.

 

[Peace River]

Is it the saltwater bucket? Is it possible that this is the binding agent from the salt mix? This is common when using Instant Ocean salt.