Will cyno in sump spread to main tank??

Hard to say. I had 2 65 gallon and 1 50 gallon tanks plumbed together--one was a reef, one was a FO, one was invert only. The FO had cyano, neither the reef, the invert, nor the sump ever had any. Not so much as a spec. I suspect that some organism that consumed the cyano was being eaten in the FO, but thriving in the other tanks. Snails? Worms? I never knew for sure, but think it was something crunchy, since the FO had lots of pods and worms, but limpets and snails were favorites of the trigger and puffer. I'd remove as much of the as you can--you should be able to just swirl the bits off in a tub of old water, no need for drastic measures. If it shows back up, then be concerned.