Brown slime in hot water tank in loft

I went in the attic to have a look at the cold and hot water tanks and noticed that the hot water tank had some brown slime at the bottom and along the sides. It looks like like a gooey paste and it makes me feel sick that this dirty water is being fed in my hot water cylinder (not that I drink from the hot water tap but it just feels disgusting).

Now I don't have a lid for the hot water tank in the loft but I have covered it with a plastic sheet to stop dust and other things from getting into it. What is that brown slime thing and how to get rid of it?