Algae is just a sign of excess nutrients or too much light. What is your photoperiod, and do you overfeed at all? :)
I did some googling and I think light may be a problem. I have been quite forgetful about turning off the light at night. Definitely not overfeeding. I have two bettas in this 10 gallon tank and they each get 4 pellets twice a day. I give them one at a time and watch them eat them. The tank is underplanted as well and I have some plants on order.
algae on the glass is normal and just needs to be scrubbed.
what is your light?
what ferts do you use and how often?
do you do co2 or liquid co2?
also what kind of algae are you seeing? that can tell you a lot about what you have going on!