A Dungeon Master:
You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects, and you can cause green fire to leap from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.
Tempe, there isn't a second creature to hit, so I'm not sure you should be using green flame blade in this situation. No, you would be using your melee attack modifier, then just putting damage on the next creature. I'll go ahead and allow the bonus damage to apply to the crab because in pbp it's extremely rare that we will ever know when the bad guys are 5' next to each other. So +3 to damage whenever you use it.
I've been doing it right (err on side of worst outcome for me...
The spell didn't hit, just the +3 great sword.
At this level GFB adds a die to each; the initial damage and the Casting Modifier delivered to a different foe.
The REAL reason to cast it though is because it is a fire spell. I have a feat that aids fire damage and causes a very minor fire shield when I cast fire spells. If the crab attacks Tempe then it takes 1d4 fire damage(probably not counting as a spell I cast so... Your call about trolling the 1st 1 on that).
