What can cause swimmingpool water to have green on pool wall?

Sounds like plant growth like algae or moss.  Not enough chlorine?

I agree with NJoy. Sounds like algae. Call a professional pool cleaner.

That's called algae, and to prevent this keep changing the water every week or at-least 2 times in a month.

Carl, you do NOT change the water in a pool every week. That is the most ridiculous answer ever. Do you know how many gallons of water are in a pool? Do you know what your water bill would be if you were constantly emptying and filling your pool every week? Unbelievable!!

Carl may not have a large pool anonymous.

Unless Carl has one of those pools you blow up with a pump, it is simply not feasible to drain and fill a pool every week. It is ludicrous. And in many areas it is not even legal.

I agree anonymous, but Carl didn't say what kind of pool he was referring to, and you would think one would assume that he was not speaking of an in ground pool from his answer, and therefore not react the way you did to his answer.  It is better to think and re-read, (if need be) than to jump down some one's throat so rudely, the way you did.

Well, whether it's an in ground or above ground, it is still not feasible to drain and fill a pool every week. Even those pools you get at Walmart for $200 can't be emptied and filled every week. Only the little kiddie pools could be, and I just doubt  he was talking about one of those. Sorry, but Carl's answer was still ridiculous.

BTW, that wasn't THAT rude of me. That was only a 2 on my personal rudeness scale. Now the guy who said women who breast feed are child molesters, he got a 9.5 from me.