A large Channel Lock type type pliers set to a wide enough opening to not damage the union but grab it firmly , or a strap wrench, should be able to get that open. If it is just the O ring under the union, it should be able to be cleaned, lubed and reinstalled and the leak stop. If that doesn't fix it, likely the whole body needs replacing, not that hard, but more expensive. Usually its just the O ring.
Would be good to get all that debris away from the motor so the cooling fan can do its job properly.
Depending on the age of the pump, you might be able to get it covered under warranty. If it's not leaking at the gasket and leaking through the pump housing it's a manufacturing defect. Even if it's out of warranty you should be able to replace just that part and put some new seals in it and be good to go.
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