DO NOT use Ocean Forest to start seedlings! FFOF is a wonderful soil, for plants that are more mature. Definitely high risk of burn when used by itself to start seeds.
Next time use something with virtually NO fertilizers at all. There are many products available that are explicitly labeled for seedlings. To be certain you should check the ingredients of the products you buy.
Fox Farm Ocean Forest: composted forest humus, sphagnum peat moss, Pacific Northwest sea-going fish,crab meal, shrimp meal, earthworm castings, vermiculture compost (bedding material and manure), sandy loam, perlite, fossilized bat guano, granite dust, Norwegian kelp meal, and oyster shell (for pH adjustment).
What is in bold there are all the things that would be providing nutrients and for cannabis seedlings this is extremely likely to be excessive.
The base mix I use for seedlings contains: sphagnum peat moss, perlite, dolomite lime, and an organic wetting agent. Which is Black Gold seedling mix, Sunshine Organic is the same thing and is also widely available. To this I add coir and some earthworm castings, nothing else.
You could (should?) have flushed the instant you noticed the yellow spot, or best case scenario gotten them into a less potent mix. I would still recommend that if they haven't shriveled up entirely yet. There is a chance it could live through this, but it is obviously a major set-back for the poor thing.