Your jellyfishlike creatures are probably the medusa stage of some kind of hydrozoan. Many hydrozoans undergo alternate metamorphosis: A sessile polypoid form releases free-swimming medusae, which produce eggs, which in turn develop into the polypoid form of the organism. (The common hydra is a hydrozoan, but it is a freshwater organism and lacks a medusa stage; it reproduces is entirely by budding.)

http://library.thinkquest.org/26153/marine/sketch/600.jpg

Hydrozoans are common worldwide and could have entered your aquarium along with any local marine life, rocks, or seawater you may have added. I'm specifically thinking of your hermit crabs. Their shells are ideal places for hydrozoans to settle, and they would have had to be kept moist during transport. In fact, one hydrozoan species, Hydractinia, grows on hermit crab shells in such numbers that a symbiotic relationship might exist: The hydrozoans protect the hermit crab from predators, while the crab's messy feeding habits provide an open buffet for the hydrozoans.

Because of their defenses, hydrozoans have a limited number of predators. Some sea slugs consume hydrozoans, and many sea slugs are also quite attractive.

http://www.biology-blog.com/images/blogs/Sea-Slugs-32132.jpg

If your mystery jellyfish really are hydrozoan medusae, you may want to try to collect a few for identification. There may be a specific natural predator for your particular species of hydrozoan which could make eradication very simple.
Source(s):
http://en.allexperts.com/e/m/me/medusa_(biology).htm
http://www.ucmp.berkeley.edu/cnidaria/hydrozoalh.html
http://www.reef.edu.au/asp_pages/secb.asp?FormNo=18

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