It can be a number of things, and since it is in an area that is of great importance, you should visit a urologist at once.
I honestly don’t know what this is, my guess is some type of cyst, which could be malignant but more likely is benign. I think it may be a good thing it’s not attached to the testicle, if it needs to be removed it prbly won’t affect that testicle at all.
However, I’m not a Dr. nor a medical person, but even a Dr. would tell you they can’t diagnose this from a description, they need to examine you physically. So your best course of action is to make an appointment w/ your reg Dr. 1st, let him/her examine you, you may be referred to a specialist (possibly a Urologist), get diagnosed & have a plan of treatment.
My guess is that they will want to surgically remove this. Most likely it started very small & has been growing, so even if it’s relatively harmless, it’s going to start crowding the testicles & cause scrotal swelling & discomfort, so I think you’re going to want it removed, regardless of whether it’s benign or not.
Please do not delay - make a Dr. appointment ASAP & please check back in & let us know what’s found & what treatment they recommend. We can all learn from your experience...