sand covered in purple gunk WHY?

[Canadian]

Your "White Sand Stirring Star" just obliterated most of the fauna you had in your sandbed. Congratulations! Now you have nothing in your sand to consume detritus other than your star fish and some bacteria. The reason it was "stirring" your sand was to find things to eat. So now you've got next to nothing to consume detritus in your sand and your sand bed's ability to process nutrients has been seriously compromised - hence the cyano outbreak. Lose the star fish . . . it's simple.