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Where exactly is the lump? If you lift the tail, where is the lump using a clock face as your reference (example; it is at 4 o'clock)?
Is it open and bleeding?
Is it firm or soft?
It sounds like Moose has an anal gland abscess that has ruptured. There are other less common conditions that can look like what you described, but the abscess is probably the most likely. I am including a link about anal glands and their potential problems.
If it is an abscess, it will need to be flushed out, and Moose will need to be put on antibiotics. Your vet may need to put a drain in to help clear out the infection as well.
Good luck and I hope this helps.