I wouldn't know why, there's no 'yuck factor' on algae I'm familiar with, certainly not on a creepy-crawly level. They certainly don't care if you use it as animal feed. Kelp isn't a huge part of the western diet but its certainly available and I've never seen any react with disgust at the concept of eating it. As to lab grown meat, that's very wait and see, its too new and too R&D phase to deal with. I doubt most people would object to finding out their steak was grown in sterile, clean, meat-growth facility. It's all image and marketing though, one could make them appear as shops of horror contrasted to scenes of prisitne pasture and happy cows grazing, alternatively one could contrast screaming animals being slaughtered in filthy conditions to sterile white labs with no pain and suffering and loss of cute mammal life attached. But algae? No, people don't care, its already in the accepted direct human diet and nobody cares of their cow was eating kelp rather than corn, unless it effected the taste.

Personally? I enjoy and consume meat, but prefer less of it then most Americans as a percentage of my diet, my older sister and I were raised as 'mostly' vegetarians, but it didn't stick very well on me and barely at all on her. In terms of ethics, I don't think higher meat consumption is much to be worried about. Vegetarian for ethical reasons of killing a thinking creature, or consuming less meat for health or economic reasons, or simply not liking meat, those are all fine by me. Encouraging others not to though starts getting into the territory of what I normal dub 'Malthusian Cultists', who are people I generally feel are guilty of technophobia, especially if they advocate we actually decrease the human population, in which case I consider them guilty of inciting mass murder.

More broadly though, the meat vs plant argument will inevitably lead to maximizing resource efficiency in general, and that has a very dark zone when explored, even without getting very sci-fi-ey. Its also where a lot these so-called rational sorts prove they're more cult then scientist. Consider strawberries, you here complaints about beef and meat in general, but never strawberries. 10,000 pounds per acre is a very good yield for them. However, not only do they require absurd amounts of labor compared to meat or cereals - labor which burns many calories - but a strawberry only has about 150 calories a pound. So that's an annual yield of about 1.5 million calories an acre, very low by modern agricultural standards. And that without even factoring in all those calories burned picking the stuff, which is huge compared to something like corn or beef. A 10,000 pound/acre strawberry harvest generates about 2 man-years of food, a 160 bushel/acre corn harvest produces 20 man-years and does so with an entire order of magnitude less labor, and indeed you'll get more meat calories then strawberry and still with less labor. Yet no one complains of them, and strawberries aren't even a bad crop as these things go.It like people who encourage others to ride a bike or walk to work, not realizing that the person will burn up more fossil fuels in terms of the food they ate then just driving a car. Riding a bike to work for the exercise and fun is a different story, much like eating healthier foods you also enjoy.