I've noticed dead coral is the first place cyano and then other more tenacious algae starts. It seems like there's something within that matrix they can extract that they can't get as easily somewhere else.

Crushed and disbursed amongst your sand might not be as bad, but it would be pretty easy to spot test.

indeed :) most beach sand is made this way......here is a school of bumphead parrotfish I photographed doing just that

https://farm3.staticflickr.com/2833/...29dd4d41_b.jpg
https://farm3.staticflickr.com/2839/...cfe2a066_b.jpg