What is this?! Brown on main stalk!!

This is my first grow, I'm very worried,

today i saw brown spot on the main stalk this is an auto-flower 29 days old, organic soil, I really don’t know what is wrong with it. she used to be happy now she’s kinda droopy a little bit and slow grow.

I have another auto, she looks good and happy, I’ve been watering them both at the same time

i water the plant every two days with little water because I’m worried to overwater.

Please help me
Thank you

You shouldn't worry, your plants look fine and lovingly cared for.

From growweedeasy.com (a solid website full of growing tips and information):

If a stem is totally exposed to strong direct light, it will often turn red, pink, or purple over time.
This is a defense mechanism by the plant to protect the stems from too much light.
This type of red coloring is especially common when using grow lights that produce UV light or when using training techniques that expose the stems to light, such as manifolding and defoliation.

Click to expand...

You obviously haven't trained this plant, but since it has so much internodal spacing the stem is in fact more exposed to light than "normal".
Again, don't worry, your plants look healthy.

If you want you can try and keep your grow light closer to your plants to decrease stretching.

It’s a healed injury. Don’t worry about it. If you supercrop by twisting the stems along their lengths as opposed to snapping them over you will see these a few days after you twist them.