Algal leaf spots can be common on plants such as crotons, ornamentals, and evergreens. The spots are usually grey, green or tan. Poor circulation can contribute to algal leaf spots. Look for it on older leaves.
Damaged leaves won’t improve, but newer leaves may be protected by applying a copper fungicide. Under a microscope, the spots look like snowflakes.
See this link for photos of algal leaf spot on magnolia: https://plantpath.ifas.ufl.edu/u-scout/magnolia/algal-leaf-spot.html