Does it look like this?
https://ysjournal.com/the-humble-immortal-hydra/
If it does, I had that in 2 tanks, if you send me a message or email
[email protected] with your address I have some SL Aqua Z1 which worked for me and I can send you some as it is internal post. It won't get posted until this weekend if you are happy to wait until then but a friend is coming who can post it then! It is a white powder, directions as per this link, really easy! It is a natural nut oil so is harmless to shrimp and 1 scoop will be sufficient in your 20L.
http://www.sl-aqua.com/en/conditioner/products_z1.html
Hydra normally come on plants and as I say I have had that even from professional shrimp suppliers that I have got plants from! I have (many years ago) bought quite a lot of shrimps on ebay and I never had a problem. The best way to be safe with plants and shrimp is to quarantine them, if that is possible, but for now we are where we are so we just need to get you sorted!
Simon