Hitchhiker

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At that size he probably feeds on phytoplankton. I would collect him in a jar and move him to a smaller tank and keep him in plenty of phyto. Match your params exactly if he appears happy.
 
Not sure if its the same species, but look for polyps such as shown below within cryptic areas of your tank. It most likely eats very small zooplankton (baby pods and such) and particulate matter as I am not aware of any jellys that sting, trap and eat phytoplankton.

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- Creating a jelly

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Jellyfish & Hydroids

Chuck
 
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