It looks like there is just a hole in the wall there:

http://photo.tepco.co.jp/library/120120_2/120120_10.jpg http://photo.tepco.co.jp/library/20120810_01/120810_01.jpg

(The damage lines up pretty well across those photos)

Doesn't really explain why they masked it out though.

It could be anything, like a pile of garbage with a coca-cola vending machine on top of it, or just a piece of equipment.

Or, like in a video posted in this thread, an unsuitable writing on the wall, there seem to be a lot of them: http://i.imgur.com/0QL7a.png (similar size btw)