I agree with floral_uk, I can see the mold in your picture you posted. You need to start completely over with new soil-less media and a sterilized pot. Then, if you use the same area to grow them, I would also water with either....diluted H2O2 or Chamomile tea. (stop over watering, if you are new to growing....error on the side of under-watering.) I personally use diluted H2O2 for all of my indoor growing to prevent damping off. It also has the added benefit of aerating the soil. 1 or 2 T, of 3% H2O2, per quart of water.

Also, a fan couldn't hurt. Set on low at least a few hours a day. (I do it after watering) It will help to produce sturdy seedlings and dry out the top layer of soil to prevent mold/mildew.

Always be thinking about ...light, temperature, moisture, wind, and nutrients.

I can't work out why you are sowing this now. I sow coriander directly in drills in May. It never transplants well when grown in pots. Where exactly are you?

I agree that cilantro should be direct-sown in place!

Thank you all for your answers! I am not really in the states, so the climate is a bit different. I will re try with all your suggestions, got a bigger pot to be able to seed this directly.

Once it sprouts, if you can get it outside, or @ least where there is good air circulation, very bright light & lower humidity, that can make a big difference. As suggested above, a fan may be helpful.

Now is the time to be starting it here in FL as well. Summer is way too hot & humid!

Andrea, do you need to grow in a pot? they'll do better if you sow them directly in the ground. If you must use pots, IME, they don't have deep roots, so you don't need a deep pot -- in that case, I'd just put the pot outdoors and sow.