Lump on Mufasa

A few weeks ago, I noticed a lump on Mufasa's side. I don't think its grown any bigger though. (its a little smaller than two fingers wide and seems attached to something as it doesn't move around)Its located by his last rib.
I'm wondering if this is something to worry about, or if its normal for older cats to get lumps??

I would take Mufasa to the vet and have it checked out. It doesn't sound like a cyst because cysts are round and movable.