What is the cause?

[zeitate]

Have you seen anything like this? Only the top part of the stem has dried up. This is a 1 week and a half old seedling.

 

[Terry385]

Look like you flex sealed it
https://www.getflexseal.com/?MID=6440777

 

[Bugeye]

Sorry, it is called damping off. Your seedling is not going to survive. Did you dome your seedling or keep it in a high humidity area, or keep your medium really wet? Really damp conditions greatly increase your odds of getting it.

 

[zeitate]

Sorry, it is called damping off. Your seedling is not going to survive. Did you dome your seedling or keep it in a high humidity area, or keep your medium really wet? Really damp conditions greatly increase your odds of getting it.

Yes, it was kept under a humidity dome, where humidity was like 95%-100%. And it is in an aero cloner.

 

[dirtsurfr]

Lights too close?
Keep em high the light will reach them just fine from 6' away.

 

[Bugeye]

Yes, it was kept under a humidity dome, where humidity was like 95%-100%.

Been there, done that. Intuitively it feels like putting a dome over them should help them, but with mj, domes are only advisable when cloning. Seedlings are especially prone to damping off so go with no dome and a light fan blowing air across them. Once the stems harden they become more resilient to it.