[garfieldnfish]

I have 3 otos, one common oto and 2 zebra otos. The common oto (different tank) ate blanched zucchini and cucumbers right away. When I got the first 2 zebra otos, one started to eat the zucchini, the other never did eat anything that I could tell. He died about 1 month later without ever touching the veggies or algae wafers.
I bought another zebra oto, had him in quarantine for 2 weeks, and he never ate anything that I could tell. Once I added him to the tank with the other zebra oto, he hung on the veggy clip with my resident zebra oto within a day. Looked to me like one oto told the other "Look dummy, you can eat this stuff". Both of the zebra otos are doing great. Give him some time, but none of my 3 otos eat algae wafers, they prefer veggies hung on the tank wall using a veggy clip.
Most likely, if he is sucking on the tank wall, he is eating algae too small for you to see. If you just see him hanging on the wall and not eating at all, keep an eye on him. But if you just got him yesterday give him a few days to adjust. (And possibly a friend of his own kind, they like company)