What's this wart-like bump inside my dog's ear? Should I worry?

In the picture, it looks like a mass to me, so I would recommend going to Rey's vet to have it sampled (quick, easy, no sedation required) and the cells looked at under a microscope to see if it's something concerning. Young dogs can get a benign mass called a histiocytoma that goes away on it's own eventually, which would be my first guess. Your vet might say it's a wart when they see it in person, in which case no sampling is needed, but keeping Rey away from other dogs until it's been gone for 2 weeks is a good idea because the virus that causing warts is very contagious. So, you don't need to be highly concerned, but should have it looked at.