First, I believe that the "black circled things" are Limpets. They will show up in the hundreds, but most don't survive for long. Then you'll have another outbreak of them. They're harmless. And the jellyfish things are just that, jellyfish. They won't live for long in your tank. Do you use NSW?
The jelly fish things seem to present mostly in the morning when the lights first come on and then disappear during the day. If the NSW question was do I get my water from the ocean the answer is no, I use a mix Continuum's Halcyon