Brown sludge/scum floating in CH header tank, is this just bacteria ?

[DIY7747]

I was about to drain the heating system and noticed a brown skin floating on the top of the water in the header tank. there is also some white 'salt like' colouring. I assume this 'scum' is a sort of bacteria that needs to be removed before draining the system. Can anyone advise what it is please?

 

[AWheating]

its what we call in the trade and some crap or crud

its probably system contaminates or dust from the loft.

if the tank seems quite dirty it would be better if you drained it first to stop any crap getting into your system when its drained, then give the tank a good clean out. before you refill the system.