Clean it up and fill in any voids with a two part epoxy. Slap a new piece of aluminum channel on it and let the four winds blow.What year is that hull?
What you're seeing is the inner liner coming up and over the top of the transom. I agree with CB, clean it up with a small grinder, mix up some thickened epoxy, squeeze it into the gaps and cover it so you can't see it. The epoxy will protect it. This separation is very common on these boats since they make the hull, set the stringers in, foam the stringers and maybe elsewhere depending on the year, lay a bunch of bonding compound everywhere the liner will touch and lower the liner in.