7 Year Old Rottie with Bladder Issues

Hi everyone, I'm new to this site but am hoping to get some advice. My rottweiler just turned 7, and he has been having bladder control problems for the last year. We brought him in several times because he suddenly started having accidents in the house, along with cloudy/bloody urine. He was diagnosed every time with a UTI. Just today, he urinated outside on our patio (he couldn't even make it to the grass) and I noticed (TMI) what looked like yellow mucus or puss mixed in with it (about four table spoons or so). His other symptoms include trouble walking (which the vet said was from his "knees" going bad), as well as several black growths on his paws and around his elbows. My mind is automatically thinking cancer, but I hope I'm just assuming the worst. Anyone have any insight or experience?

I'd be back at the vets insisting on a full range of tests, I have never seen anything like your photo but it could be something treatable, you just need a proper diagnosis.

Thank you for the response! I called the vet and they want us to bring in another urine sample. We're now looking for a new vet because his current doctor just runs the same tests over and over, and we're getting frustrated.