When you see/hear someones wealth and happiness, you also become delighted about that – that quality is called Mudhitha. This is the opposite of jealousy or Issa.
This is what is to be found on the Abidhamma book about Mudhitha.
අන් අයගේ සැප දකිනු කමති වීමේ ස්වභාවය යි.