just a thin string of poop (liquid) hanging down from my bird's vent. I removed it and it was actually poop, not anything else. I will keep watching if she has it again.
Anything to worry about?

also, 2 days ago somebody pooped this pile at night or in my morning, when I saw it, it was too late to identify who it could be. Looks like a huge pile of poop - rather unusual.

A string hanging down happens once in a while. It's not a sign of a problem unless it happens a lot - in that case the bird might be constipated or have some other problem getting it to "let go".

Huge poops usually occur because the bird has been holding it in for a while. This is usually associated with nesting behavior, since parent birds don't poop in the nest and usually don't even like to poop close to the nest. So keep an eye on the adults to see if they're acting nesty.

I am watching Stella, who had a string in her vent. Not actually a string, it looked more like a poop that stretched down as a thin string and couldn't be dropped for some reasons. She didn't have it anymore, I watched what she pooped at night, and it was normally shaped poop but maybe a little more green (emerald shade) than I would like to see.
I think she behaves a little bit under the weather, but just a little bit (or maybe she is concealing her sickness).

Isn't there a kit people can buy to quickly test their birds' poop themselves? like strep throat kit?

I've never heard of such a product, and the internet isn't turning up anything. You probably need to go to a vet to get it tested. If you looked around you might be able to find a lab where you could send a sample for them to examine.