Try PoolRX. It is easy to use -- you simply toss the PoolRx basket in the pool and that's it, let the product do its job! PoolRx is an EPA registered algaecide and chemical reducer. Its places in the skimmer or pump basket and lasts for 6 months. The minerals form a stable water environment that not only eliminates algae it also prevents future algae from growing. The product comes in 2 sizes, one for 7,500 to 20,000 gallon pools and one for larger pools. Keep in mind that although this product has a larger upfront cost than liquid algaecides but 1) it lasts 6 months so it ends up being cheaper and 2) it works! PoolRx also helps save your chlorine, phosphate removers and other chemicals.
We ran a pool service company whereby we cleaned and maintained commercial and residential pools and at one point we had over 500 accounts. When our customer's pools had stubborn algae we used PoolRx. We speak from experience!