Nice pics btw, too bad it has to be on a problem but you'll be taking awesome pics of crazy colored SPS in no time. I think it's diatoms and here's why:
- In my experience, cyano grew much more like a "mat" and there weren't that many long stringy things. When I had diatoms, they were more spaced out and stringy.
- Diatoms make sense given the age of your tank. It's been running for approximately 90-100-ish days? Seem like you're right on track for some of the new tank uglies like a diatom outbreak followed by maddening GHA.
- When I've dealt with cyano, it's been red. So many of the things we deal with like this are brown, brownish, or rust colored that it's hard to even describe but the cycano that's I've dealt with was red.
- I usually see dinoflagellates with a lot more bubbles trapped in the slime stings. Is this always the case? Of course not, that would be too easy.
- You've been dealing with this for a little while now (at least) and it's not too bad yet. One things I've noticed about dino's is that they reproduce at an astonishing rate and it's surprising how fast they can explode even in a single day.
The biggest factor in why I would think it
might be dinos is the super low levels of nitrates and phosphates. Dino are always in our systems but they typically get competition from all of the other less-evil stuff in out systems. When the nutrient levels are very low, some of this competition can die off and dinos start showing up.