I am no expert on fuchias, but I have seen this kind of thing on a number of other kinds of garden plants, and even on a number of florist flowers at the shop where I work.
Developmentally, flower petals and sepals are modified leaves. Sometimes the development of the parts in the forming bud 'glitches' to one degree or another, and you get an odd mix of flower parts that are more leaflike. This can be as simple as one petal that is odd-looking and greenish, or it can be an interesting mix-up like your flower in the picture.
There are viruses that can mess up development like this, but more often, it occurs naturally in single flowers. Since the other flower on the plant seems to be developing normally, I strongly suspect that your odd flower is just a really cool-looking anomaly