Most alage eaters do not survive solely on surface alage and require extra food in the form of algae tabs and some fresh vegetables ( zucchini/courgette is often used, as is cucumber )
However, your tank is not big enough for any sort of algae eater apart from an otocinclus, and they are often hard to keep alive, and prefer to be in groups ( which you do not have space for in a 5 gallon ) .Most only last a few months before expiring even with good feeding.
Your 5 gallon is really only big enough for your betta.
The main problem I'm seeing here is the " I can't clean it very often" attitude. Firstly, there is no fish on earth that will magically clean a tank for you, that's your job, and a 5 gallon should not take more than 10 mins to scrape off a bit of algae.
Secondly, if you are getting algae then you are either overfeeding ( the bits left over are what the algae feeds on as the food rots ) or have a light that is too high.
Most commercially available 5 gallon tanks come with pretty low lighting so I would have a strong guess that your are overfeeding the fish . 2-3 pellets or a small pinch of food once a day ( or better yet, every other day) is all he needs. A couple of live or frozen bloodworm at the weekend is also a nice treat for a betta.
If the tank is sitting near a window where sunlight can hit it, then this will also cause an alage problem.
Also, you should be doing a 50% water change once a week every week.
That takes less than 15 mins out of your week, and no offence but if you can't manage that then you should re-home the fish if you haven't time to take care of it.
They are not hard to care for but at the same time require more care than you might imagine.