wow, not sure what you guys have been doing with your cpu blocks lol!. Cammy, you do know that the internal plate hasn't been sitting correctly inside your block? - Look at the watermarks/stains on the silver plate - they're off centre!
Had mine over a year and I recently cleaned it out (about a month ago) - theres was only an ever so slight build up in the internal fins but nothing as major as the top post or as bad as the post above!
tbh - this has happened with all cpu water blocks I have owned. Theres always a bit of crap in the water or other components which gets flushed into the tightest spot (i.e. CPU block fins) - nothing to be worried about tbh (unless like the OP!) as its usually recommended to clean out water setups every say 6 months or so.