The only thing that I have seen that looks like that is a cycling system for an in-floor cleaner.. Under the dome is a device that open one of the pipes at the bottom in a sequence..
Not saying that is what it is, but it looks very similar.
Show us some pics from further back so we can see how it is connected to all your pool plumbing..
Do you have some type of pop up heads in the bottom of your pool?
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