It's the impeller-magnet from my skimmer. Split open apparently. I serviced the skimmer about 4 months ago, but today after some 'maintenance' it wouldn't start up...
That is the exact same pump and looks exactly the same as what happened to a friend/client just last week. I saw him yesterday and he said that he couldn't find just the impeller in stock anywhere and had to call the company and after explaining that was given a discount to order the entire pump.