I have some potted tomato plants and the soil just started turning yellow on top. It is spreading over the whole top of the pot.
Yellow Soil Potted Tomato Plants
Certified GKH Gardening Expert
You have some type of slime mold forming on your soil. While this will not affect plant growth, other than shading out light, you may want to consider repotting with fresh soil. Here is more information: http://msucares.com/pubs/infosheets/is1684.pdf