The moss in the gutters probably trapped dead leaves and such which have rotted down to form a wet, slimy layer in the bottom of the gutter. The water in your rain butt should not be harmful to plants generally, more like a weak liquid manure, but you would have to be more careful with plants that have special requirements.

Might contain some bird droppings washed down from the roof... so whatever is in bird-droppings could be in your rain water in a weakened form...

~ is the answer to turn the tap full on and let it empty and then flush through with disinfectant or to use my other butt for my proteas and use this one for everything else until it is empty assuming it will not harm my other plants?
~ my husband has put a pan scourer in a plastic ball at the top of the drainpipe to filter the water so I am bit concerned that the water butt may now have a layer of sludge which may be difficult to get out as I thought it would be trapped ~ is there any product to clear the water?

I've heard of people putting 1 leg from a pair of tights over the drain-pipe to filter anything unwanted. When the moss 'balls' fall from our bungalow roof onto the path, we find the birds pecking at them for 'bugs' so not really sure what could be in the water !

If it were my water butt, with lots of rain forecast, I would be emptying it, cleaning it as best I could, and then letting the rain fill the butt up again, so that I had nice new water ready for the summer....

I'd be tempted to empty the butt as we're bound to have enough rain to refill it before summer starts? Esp if you've got a 'spare' to use in the meantime.