Definitly looks like cyano. Dinoflaggelates are brown and forms strings when it gets longer. I have never had cyano grow long strings, always a solid mat. It will grow on the substrate after syphoning as you are desturbing the sand and getting some detritus to the surface. if you take a tukey baster or a power head and direct it onto the rocks, you will see a lot of detritus being blown into the water column. But the cyano should then take some time before popping up again on the rocks. Substate is a bit ,pre difficult and you will have to vacuum it often to get the detritus out.
What worked for me is to syphon into the sump and add some filter wool or filtersocks ( I actually did both) to catch as much as possible. When you vacuum during a waterchange you have limited time. With you syphoning into the sump you can carry on for much longer to make the most out of your effort.
