To me that looks like liquid had beaded on the end of that seedleaf where it cups a little and caused damage -- sunlight or maybe the liquid was fertilizer or something?
Worst case, fungal infection due to the moisture or nutrient-rich solution. If you don't want to take any chances, clip off and discard that seedleaf to prevent spreading. It already has a true leaf, so it will manage without. Tiny sharp tipped (and curved) cuticle scissors or embroidery scissors... or dissection scissors are useful for tasks like this.
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