My plants have been flowering for 20 days. I found this on one of them (Pic). What’s this. Should I remove the plant from the garden
And why did this happen
Yes it is. Unless you want pollen seeding your plants then yes remove it. Doesn't look like it's released any pollen yet so you should be ok. Why it happened? Can't say.
Yes and it looks like it's about to release pollen. Check them all. If they're all like that then pick off what you can and finish them out. You'll end up with some seeds but the weed will be smokable and could be quite good. If it's only a couple and you have a bunch of plants then remove the hermies.
If you start removing plants you may have none left by the end of flowering. Carefully tweezer off the sacs you can see and carry on as usual. Seeds arent the end of the world and it happens mostly to newer growers or stressed plants imo