Yellow spots and brown stem

The soil you have it planted in looks too heavy to me...like it needs more perlite. These things need a very light and arid soil to thrive...like 60/40 soil/perlite. Also, I suspect your issues are coming from over watering...the biggest killer of plants by new growers. LOL These things need defined wet/dry cycles to be healthy and happy. So, if you've been watering this plant before the pot has dried out to at least 80-90%, you're watering it way too much. When you first sprout a seedling or start a clone, nothing is going to happen much in the foliage department until the plant forms it's root structure to it's satisfaction. Once the plant is rooted in well, you will see the foliage start to grow and it will start to use more water. But in the beginning, nothing is happening too fast and it doesn't use much water at all...which is where most new growers get into trouble. If the pot doesn't feel like it did when you loaded it with fresh soil, then it's NOT time to water. The pH range of the water you use is extremely important as well. You should adjust the pH of any liquids going into your plant to a range of 6.3 to 6.7. Continue watering/feeding with liquids out of range will lock up the roots of your plant and it can't take in any nutrition. Right now your plant doesn't look too bad. I suspect the issue is from over watering...and the stem is not brown, it's purple..and nothing to worry about. Figure out how to water and get some more perlite into the soil you're using. Also, that soil might have something to do with the yellowing on the leaves too. Lots of the soils you buy that say organic actually are loaded with slow release fertilizers and can burn a tender young seedling. Nothing to get up in the air about. They will typically work out of it and come on out and do well. Just watch the watering habits, load some more perlite into the soil you're using and manage the pH of your water. You shouldn't need any nutes until way on down the road after the plant has had a chance to use what naturally came in the fresh soil. After that, then you start to give them nutes at an even more diluted dose than what the manufacturer recommends. They, of course, want you to give lots to your plants because they get to sell more. But I've found that it actually takes a lot less than they recommend to grow healthy happy plants. Good Luck!! TWW