Chocolate for topping didn't melt
Assuming the chocolate really hasn't melted at all, I would just remove it and melt it seperately before pouring it back on top so it can actually combine with the marshmallows.
If it sort of fused with the marshmallows, I would consider getting more to just put on top, depending on how well the not-melted chocolate covers the marshmallows. If the newly melted chocolate can't reach the marshmallow, it'll still just fall apart after the 2nd batch of chocolate cools.