Heck, I don't even know the terminology of a half union. But I do know what he's talking about. There's an adapter between the pipe thread on the pump and the hose. The goop you see is probably a pipe sealant. You may have disturbed that connection. You should be able to loosen a hose clamp and work the hose off...some tricks to do that. Then the union/adapter can be unscrewed, threads cleaned, pipe sealant reapplied and put back together. Yes, a pic would be worth a lot.
Al