How old is your tank? Over time the aragonite sand can become saturated with phosphate. The cyano will grow in the places with the lowest water flow over the sand bed. Even if you think you have good water parameters the cyano NEEDS two things to grow, nutrients and moderate to low water flow.
You can help battle this by vacuuming the mats up as soon as they form, and even leaving you lights off for 3 days. Make sure that you are filtering well during this. The cyano can release a lot of crud in you tank if it does off.
As I said before it is not uncommon for this problem to worsen as your lights age, and approach 6-8 months when they need to bee replaced as well.