Someone solved the slide mystery!
The answer: “That’s a slide of human blood cells. The pale gray objects in the background are red blood cells (erythrocytes). The larger, more colorful cells are white blood cells (leukocytes). The one by the arrow looks like a type of white blood cell called a lymphocyte. The one above it is probably a neutrophil. By the way, all of the cells seen here have been dyed (stained) to make them easier to visualize and identify. Now you know!”
Thanks to Peggy Rolfsen!