The shock you are using is Dichlor and for every 1 ppm Free Chlorine (FC) added by Dichlor it also adds 0.9 ppm to Cyanuric Acid (CYA). Whereas the chlorine will get used up, the CYA does not and builds up over time making the chlorine less effective and it more likely for you to get algae. The early stages of algae make the water look dull, then cloudy, then eventually green, but it takes at least 2-3 days.
I suggest you get yourself a good test kit, the Taylor K-2006 from Taylor here
or from Leslie's here
or from other online stores (most pool shops do not carry the K-2006, though some carry the K-2005 but that is NOT the same thing and has an inferior chlorine test) or you can get the even better TF-100 that is based on the Taylor kit from tftestkits here
. These kits use the FAS-DPD chlorine test which can measure chlorine levels up to 50 ppm and has a resolution of 0.2 ppm or 0.5 ppm depending on sample size. The test kit also measures pH, Total Alkalinity (TA), Cyanuric Acid (CYA) and Calcium Hardness (CH).
I suspect your CYA level is high so if you don't keep a higher Free Chlorine (FC) level, then you will be at risk of getting algae more and more. If you don't want to lower your CYA level, which can only be done through dilution (partial drain/refill or more frequent backwashing), then you should use PolyQuat 60 algaecide with an initial dose and then a weekly maintenance dose. That will keep away the algae even with the chlorine level being too low for the CYA level.