Sorry for the lost WXB! I know how you feel because I just lost one of my frag for the exact same reason. And a couple of my other frags which were healthy before are following the same slow death.
Many people who's struggling with cyano / dino are sometimes unsure why because these organisms are able 'fix' dissolved nitrogen gas, and take advantage of whatever trace phosphate there is. This is one reason why they grow on the acros: they extract the phosphate directly out from where they attached. The funny thing is once the acro is dead so will the cyano because there will be no nutrient left in the dead acro. This is the same reason why they grow on the sand and rock even when your N & P appear to be very low. You will be scratching your head trying to reduce N & P and at the end, it might just starve the acro more and accelerate its dead. Your case can certainly be a nutrient problem but I was trying to point out there are multiple causes when it comes to cyano and dino so you need to be well aware of what you are dealing with before taking action. That's why I am somewhat against using the special blend. It normally doesn't help or it only helps temporarily. I have taking a different approach to this problem. Instead of trying to lower my nutrient (I have been running ROX and GFO for +5 months aggressively without success), I am actually trying to increase my nutrient level and hope 2 things:
1. Another kind of algae or micro organism will out compete this pest by nutrient
and space.
2. The acro can "feed" more which would lead to healthier and stronger corals so they can fight off this pest.
If I know more or have better success in the following weeks, I will sure to let you know.
I know this but being new to SPS i know there has to be some trick someone has used in the past.
The best trick is to just understand how a closed system works. The chemistry and the biology. Once you gain the basis knowledge of keeping SPS such as flow, lighting and various other major parameters, you should have some level of success. After that, it's more or less all about observation and detect problem early. I feel like you are going very fast (for the lack of a better word). It seems like you are always in a rush looking for answer and taking actions. For example, at one point you said you are going to run carbon and then later you said you know exactly what this is and special blend is going to help and now the acro is dead. It's just the impression that you seem to expect things would turn around easily and there is some sort of switch that you can turn on and off.
Oh, the stuff you have is exact same thing I have.

It doesn't matter it's long or short. Yours is short because you don't have strong flow (judging by the picture). If you have a lot of flow, this stuff will grow very short (as in my case).