What’s this in my fennel?

I bough a fennel seedling. it was ok for a few days but then a new branch started to sprout and I noticed little green spots. I imagined it might be from the sprouting, so I left it alone. today I noticed the branch infested with this, which I imagine is a pest. I thought it was strange because it seems to be coming from inside.
I washed it and removed it manually, but does anyone know what it is and if it's possible to save it?

it does receive around 3 hours of light in the afternoon and I water it everyday (I live in a really dry city, we haven't had rain in 135 days, the air humidity is around 20%)