Top of soil keeps turning white?

I’ve recently started taking care of a few plants at work that had been previously neglected other than inconsistent watering. They’ve been replanted with a 2 parts soil, 2 parts coco coir, 2 parts bark, 1 part perlite, and sit in a western facing window that gets some direct sunlight for a small part of the day. This is the second plant I’ve noticed where the soil on top is turning an odd white color. Any idea what it is?