Feeding Algae-eaters In Quarantine

Find a glass container, a few bits of rock and a sunny windowsill. Fill the container with tank water, add the rocks and hope the sun shines. Maybe even add a tiny bit of ammonia - you want 1 or 2ppm, no higher.
Hopefully algae will start to grow on the rocks. Put one in with the otos, when they've cleaned it, add another and put the clean one back in the container to grow some more.
I'm not sure how much algae will grow in a couple of weeks but it is worth a try.

Or try sushi nori. Messy but people have found that otos will eat it.

I've read that otto's can be fussy eaters as they get older, but are supposed to like blanched veg as well as algae and wafers, ours like cucumber now and again