Was your silicone completely cured before you let salt water get to it? Ive seen salt water do some weird things to uncured silicone. If its just on the surface of it, then its probably cyano, but to me it looks like its "in" the silicone. Try rubbing it off with a finger.
I had made a refugium in a sump in the past, thought I had fully cured the silicone, exposed it to salt water and had a similar effect. It did go away however.
Whatever it is, I hope you find it and destroy!
