Sorry, I'm not sure I understand, it's displaying both numbers? or it's doing an addition rather than a subtraction?
A shot in the dark if it's a display issue... You've set a Format on your "Convert Float to String" Action. Just leave this blank.
If it's an addition rather than a subtraction, check the action on your damage dealer, make sure you're adding a negative value, or using the "Float Operator" action to Subtract.
To deal with player "Death", a simple "Float Compare" action in the "Update" state will be able to tell you if your health value has dropped below 0 and fire off a transition event to other states where you deal with ending the game