MikeATC Retired FAA, NATCA Member Nashville TN

NU,
As long as you had proper wake turbelence seperation prior to the following aircraft accepting visual seperation from the heavy, "YES" you may use visual seperation rules, between the trailing acft and the heavy, but you as the tower controller can't offer to provide visual seperaton.

It doesn't matter if the Heavy is on a GCA approach and the trailing aircraft is or isn't as long as the trailing aircraft is willing to accept responsibility for visual seperation, again the key is that you "MUST" have appropriate wake turbelence prior to the application of visual seperation