I think there is a popular belief that shocking will always kill the algae. IT WONT! Sometimes you need an algaecide. Yes your CYA lvl will hold chlorine better at around 80ppm with a SWG.
Blatantly false. You just have to bring the FC up high enough for the current CYA level. Algaecides, except for copper and halobgen based ones (sodium bromide and inorganic ammonia/EDTA combos) do not kill algae but only inhibit it's growth. Copper kills algae but the cure is worse than the disease, Chlorine and bromine kill algae (and the reason sodium bromide products work is they take CYA out of the loop) but it,too is is often a cure worse than the disease. Monochloramine kills algae (created by either adding anhydrous ammonia to the pool, which is an old time commercial pool maintenance trick or by using one of the inorganic ammonia/EDTA based 'chlorine enhancers on the market after raising the pools pH above 8.0) and also takes the CYA out of the loop but then you have a pool full of combined chlorine to deal with! So,for the most part, algaecides do not kill algae but chlorine (and bromine) does!
Also, make sure your not trying to test your water after you shock seeing as how the chlorine lvl is so high that it tends to thow off most testing reagents.
Also untrue with a caveat. High FC levels can interfere with phenol red pH testing so it's a good idea not to test pH until the FC drops below about 10 ppm but if you are using the recommended drop based kits we talk about in pool school the other tests will be valid. The TA test will not have the green to red color change but rather a blue to yellow color change since one of the indicator dyes will beach out but the results are still valid. The CH and CYA tests will not be affected and the reason we recommend FAS-DPD testing over DPD testing is that it does not bleach out and can test chlorine levels up to 50 ppm with a precision as great as .2 ppm over the entire testing range.
What type of filter do you have and when did you clean or backwash it last. Make sure that when adding something to kill the green stuff that you give the pool a quick brush down to disperse as much of the green stuff as you can. It will make the shock treatment/agaecide or whatever your using that much more effective.