Q.Apple Tree Seedlings Stunted Growth
A few months back I planted a few apple seedlings. In the beginning, they were growing really quickly and no problems. A few weeks ago though I noticed that they stopped growing as fast, in fact, they really haven’t changed in a few weeks now and they aren’t looking as healthy as they used to. As you can see in the photos, new growth on the plants look deformed and withered. I’m not sure what to do here really.

Certified GKH Gardening Expert
I am seeing signs of infection brought about by overwatering. Make sure that the containers contain plenty of drain holes to allow all extra water out, freely. The algal growth that I am noticing on the soil surface confirms this.
From here, you will need a fungicide, and to make sure that the soil dries, completely, down to about 3 or 4 inches between watering.
Here are some articles that will help:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.gardeningknowhow.com/edible/fruits/apples/container-grown-apple-trees.htm