sand covered in purple gunk WHY?

[djbh]

Josh,
cyano is the result of usually more than one thing.
Excess nutrients are one of them.Some sand sifting animals are a good help in ridding the tank of cyano.So are more frequent water changes.
The manual siphoning of the sand bed is another.
When you do siphon the sand be careful to only siphon out the top layer of the sand and not siphon out the living critters.
Lowering the tank water temp is also another one.
Higher temps seem to attract cyano.
Low alc levels seem to attract cyano also.
After cleaning the windows is a good time to do a small water change to rid the water column of algae.
You must have a nutrient export.
This can be macro-algae,skimming.
Be patient but get on it.
Is the cyano getting to a point where it creates a mat or carpet on the sand bed?Are there small bubbles on the sand bed?Does it seem to disapear or change color when the lights go out?
What is your tank parameters?
Lighting etc...
You need to tell us more for us to help.
Dan