What is up with my disgusting toilet tank?

I noticed there was a bad smell coming from my bathroom...sort of stale and chemical in nature. I clean regularly, so I started to investigate places that weren't so obvious. The only weird thing I noticed was that my toilet tank is just covered in reddish brown...rust? and there is a patchy film all over the top of the water. Looks like slime aculation or a weird growth of something? I flushed my tank to see if it would go away, and hardly any of the water went down with the flush. I haven't noticed any troubles in using the toilet, and it's not horribly old. I was just wondering if anyone knew what could be going on or what I could do about it? Anyone? Thanks!