I'm new to the nano thing and relatively new to planted tanks (started my first in February), but here are a few ideas --
I think otos generally prefer the company of other otos and a little more swimming room (I have 5 in my 29 gallon and they can be active little guys!). Other folks may have suggestions for good algae eaters. I know the cherry shrimp in my nano cleared out a lot of brown, green spot, and hair algae very quickly, but your betta might well make short work of them!
Are you using CO2 or dosing with Flourish Excel to provide a carbon source? I know that it's not unusual for a tank that has recently been started and/or recently had its lighting increased to go through a period of algae growth while you get the gertilizers and CO2 (or Excel) dosing figured out. Maybe it's just a matter of tweaking your regimen there?
I'll look forward to reading other people's suggestions, too, since I'm new to this and anticipate that I'll probably need good algae advice for my 2.5 at some point, too! Good luck, and please post what seems to help!