Many contactees claim that aliens do not eat any animal product. They claim that advanced civilizations turn into vegetarianism.
Oh, yeah?
They certainly don't catch up with the latest scientific advance of Earth. We are about to invent the technology to grow animal muscle tissue (meat) in a lab. This technology will enable us to mass-produce hormone and antibiotic-free clean meat without butchering any animals. We only have to obtain some small sample of muscle cells and we can literally call the end-product as 'Factory-Grown meat'.
If aliens are truly advanced, they can synthesize artificial meat from scratch, or culturing animal cells might be cheaper for them to obtain quality meat. Here's what I think. Consuming meat was the pivotal advance of human race that helped them increase their brain size. Protein from meat helped our ancestors to grow our brains into current size. We are basically omnivores. And I guess many humanoid aliens out there (if they exist) they are most likely to be omnivore or carnivores.
