Acid wash

#1
I've had several pools, all plaster. Keep chems balanced good. I have one small section of pool that is in shade all year due to contour and orientation. This small section is in about six inches of water and at base of pool wall near step. Looks like algae, slight color of yellow/green but can not be swept or brushed away. I wonder if it is just my eyes and lighting. Anyway, thinking of acid washing this area to make sure nothing is growing. Good idea or not? I'm draining pool next month (annually). Or try chlorine wash over that area? Never have done acid wash before, suggestions? Temp gets to low 90's in summer, 50's in winter. I wouldn't think algae would survive, so does the color, if it was there during summer? I think when I drain it I will see it for what it is but your thoughts appreciated.