Well there are 3 possibilities, it's either splitting to produce another head. It got damaged by something else in the tank. Or it's dying. I don't think it's the latter, because they usually recede from the perimeter, not split up the middle.
When looking back at your first pic, you can see the split just beginning. This lends me to think that it's reproducing. Most corals look bad while they do this.
If you are getting Cyano
though, you must have some nitrates. What is your nitrate level?