It might be worth going back to the store where you got your Otos and seeing if their store tank has algae ridden pebbles that they are munching on. If so, ask the store if you can buy a few of these pebbles, add one to your Oto tank and put the others in another container with more rocks to try and kick start your "algae farm."
Are your Otos newly purchased? If so, I would strongly recommend you isolate them in a QT, so that you can ensure they are eating and putting on weight (nice round bellies when on the tank sides, sunken bellies are not good at all). Often Otos come into the hobby weak and half starved, so during the first few weeks, it is really important to know they are eating at home and not shutting down.
Are your Otos currently getting a chance to feed on the apple, or are they getting intimidated by the Corydoras and Mollies?
Have you tried other veg besides apple, such as cucumber; courgette; sweet potato?