Brown spot on

soil or soilless... if soil, you want 6.5-6.8 pH. soilless you want 6pH ish. this is also a big pot and small plant... so could be related to irrigation practices. in this context, only water a circle around plant, but do make sure it saturates all the way down and a little runoff... wait for top 1" to dry (bit less if cococ) and repeat with a bigger circle until you are saturating entire pot with ~10% runoff. the runoff is important. it is waste water -- or toss it outside in garden, but do not let a potted plant sit in it and re-absorb over time. young plants can have blemishes... they more often grow out of these problems. as long as you apply good irrigation practices, i wouldn't worry. first 2 weeks the seed powers growth, If you have been fertilizing, take it easy for a bit. soil + ferts could be too much, whereas soilless and a ~650-750ppm mix is probably safe from get-go, but not needed as seed is self-sufficient for a bit after germination. is it possible something dripped on it? either way, if no other symptoms, expect it to look fine within a few new growth nodes.