ASCAP returns the same results as Harry’s when searching for ‘Mick Greenberg’
Mick Greenberg doesn’t exist or has written any songs besides Harry’s.
// ‘Just A Little Bit Of Your Heart' writing credits then & now.
You can listen to Ariana talking about Harry writing the song here. //
In conclusion, Mick is probably Harry’s pseudonym.
Found by germanystyles. You can listen to ‘I Love You’ here.