If you don't have returns that have circulation flowing along the walls, there can still be dead spots. It's probably even more of an issue with a pebble finish that isn't as smooth. Brushing a pool weekly is recommended for all pools.
If when you brushed this green dust were to grow in the bulk pool water, then that would indicate a problem with the FC/CYA ratio not being sufficiently high to kill this algae, but if that does not happen then it's likely to be that the algae took hold in areas of poor circulation in the crevices in the walls. Now, that said, one usually tries to get rid of algae almost completely in a pool so that this doesn't happen or happens much more slowly just from spores getting blown into the pool (so would get dealt with the next time you brushed). Since your green tint was on all walls, this means that algae was probably all along the walls the entire time and was never killed off completely.
If the walls were blue, then the other possibility would be something more chlorine resistant like yellow/mustard algae since yellow+blue=green, but I'd expect that to be more on the shady side of the pool.
The CYA test should be read with your back to the sun holding the tube in front of you looking straight down into it. That is strong indirect lighting which is what is ideal for the test to be most accurate. If you read it in direct sunlight, you will get a falsely low reading; if read indoors in not enough light, you'll get a falsely high reading. If it's really 55 ppm now, then 4 ppm FC would be the absolute minimum. That combined with the circulation at the walls might explain what you are seeing. To be sure, the CYA test is a difficult one and it's accuracy is officially +/- 15 though I believe that to be more of full scale (i.e. at 100) and that at lower levels and with care it's probably more like +/- 10 though that's still a significant range.