What are yellow, egg-yolk-like objects on top of a scab?

The yellow, egg-yolk-like objects are likely dead, dried parasite eggs. When a cat has a wound or skin disease and harbors internal parasites, eggs can be deposited on the injury; for example, fresh tapeworm segments are wriggling white rice grains that die and dry to a beige color on the scab. It could also rarely be a reaction to something the scab touched.

Have a vet examine and clean the wound, keep up regular internal and external deworming and coat hygiene, avoid too much raw meat, and maintain a clean environment.