its strange my tank was doing good till I wanted to lower my nitrates..
i tried this stuff called nitrate destroyer ... for about a week didnt notice much ... then the second week i noticed cyano bacteria and some stringy slim brown stuff ... on the sand and some of the rocks..... kinda like what you described too ... but instead of zeo im using nitrate destroyer ......
http://www.tailoredaquatics.com/index.html?a=0&d=63
i wonder how long its gonna be before the brown leaves my tank