Hi Freya
Welcome to the SFR discussions on swimming pool maintenance. There are loads of previous posts on the subject of swimming pools, and I have hugely profited from the wise words of John Withall, who I'm sure will pick up on your query.
Assuming you are sanitising with chlorine based products.../
CYA (cyanuric acid) is a stabiliser included in commercial pool sterilising products such as Chlor Choc and Chlor Lent. Basically CYA slows down the evaporation of the chlorine resulting in the chlorine to do it's job over a much longer period.
Problems arise with algae when over a period of time the CYA builds up and in effect over stabilises or blocks the chlorine. And, that's where test strips are useless, as they can indicate high levels of chlorine whist at the same time the pool water is gradually turning green! I learnt to my cost, and had to do a complete water change last year!!
You need a decent testing kit such as the ColorQ (check it out on e-bay) which will indicate the levels of chlorine and CYA, and also advise on the correct levels required. You also need to maintain the correct level of PH via PH- or PH+, as it facilitates chlor to do it's job.
If you find that the CYA level is too high, you can replace a quantity of the water until the CYA level reduces, or/and add some free chlorine (no stabiliser) such as 'domestic' javel or calcium hypochorite to correct the CYA level and halt the march of algae.
I hope that helps, but am certain that pool guru John will impart his own and greater knowledge and advice.
Good luck
Paul