Newbie needing help: Cyano? Or is this diatoms? Or something else

[raptzchel]

Newbie reefer here. Recently, I've been battling a brown-ish growth in my sand bed. I suspect it's cyano due to the presence of small tendrils, but its brown so could it be diatoms?

Its a 75G, currently stocked with 2 ocellaris clowns, 1 royal gramma, 1 firefish, 1 starry blenny, and 1 pajama cardinalfish. I feed 1/4 of a hikari mysis shrimp cube twice daily. Lights are on for 9 hours a day. Powerhead is a jebao mcp-120. Was running at 50% on the turbulence mode but was planning on bumping it up to around 75%. I had been using the API reef master test kits which were giving me 0-5 nitrates and 0-0.25 phosphates but picked up a hanna checker for phosphates (the ULR one) and two days ago was 0.5, yesterday was 0.3, today was 0.7ppm. Tank is around 7 months old.

Images are from today, did a water change yesterday and cleaned most of the sand bed in the process. Almost all of these patches returned overnight.

Thanks in advance for the assistance! I am very much new to reef-keeping and fish-keeping so any and all advice would be much appreciated!

 

[raptzchel]

Edit: Retested phosphates with the hanna checker with a fresh set of reagents. Did two tests, results came out to 0.21 and 0.14. Also tested with the API test kit for a sanity check and still registering 0-5 nitrates, 0-0.25 phosphates on that as well. Are swings of this level common for phosphates (0.7ppm this morning, 8ish hours later between 0.21 and 0.14)? Maybe my checker came with a bad batch of reagents? I do recall this morning spot feeding my corals with reef roids. I don't have many in the tank so I used around 1/4 tsp worth. Could that have caused a temporary spike, thus throwing off my testing? I believe I tested around 30min to an hour after I did that.