If you are getting otos, having fresh algae for them to munch on is a good thing, so getting a moss ball to "attract" algae is unnecessary. BTW, I've never heard that they "attract" algae. Plants will outcompete algae for the nutrients though, which might be what you mean. If you need algae, just leave your lights on a little longer, that's usually an easy way to get algae for the otos.
BTW, don't rely on just the algae in the tank to feed the otos... algae wafers would be a good idea, as well as many other soft veggies - zucchini, leafy greens, etc. Frozen peas are also a good option for the fish from time to time. Deshelled (meaning the outer shell around the soft "meat" inside the greens.)