That's a pump union and it has an internal gasket. You may be able to tighten it BY HAND a little or replace the gasket. You may wantt o open it and use gasket lube to give it achance to seal better. Using a wrench to snug it my crack the collar and cause bigger problems for you.
You are going to have to cut the plumbing. That is a 1.5" FPT X 2" MPT reducer with a 1.5" male adapter glued to the pipe, not a union.
Best to get a 2" High-Temp pump union and then put a 1.5" reducer bushing in it to attach the rest of the pipe. Will probably need to change most of the plumbing after the pump up to the valve.
Looks to me @1poolman1 is right. Also looks like this has been repaired before. You can see the extra coupling right after the 90 deg fitting on the pipe. You might have room to do the repair Poolman recommends if you cut the pipe right next to that coupling. Do it right with a proper high-temp union and you won't need sealant plus it will be removable in the future with no cutting.
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