Hitchhiker jellys are not common, so like Mike said it could either be a hydroid or jelly. I think it may be a jelly in fact, in it's polyp stage. Did you recently add anything new to your tank? Macroalgae? Livesand? Liverock? Nice pictures by the way.
If the polyp makes it so "adulthood" it can grow and start stinging your corals (and you!) and generally be a nuisance. Most don't make it past the stage it's in right now though, not enough food.
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