Greetings, welcome to the Forums.
I agree the plant pictured is suffering from nutrient deficiencies. Do apply a complete fertilizer.
As for the plant's identity, I would like to see more pictures when this plant flowers, and also when it produces fruit. Please do post again.
The name 'plum' is a common name that can be applied a wide variety of fruit trees, but the plant pictured does not particularly look a plum in the genus Prunus, which are the kind most commonly grown in temperate regions.