As Carolina Pool and HouTex stated, the white calcium nodules (which is "calcium carbonate") are from delaminated sites where the plaster layer has lost the bond to the old plaster layer underneath. There is a crack below each nodule which allows the calcium carbonate material to exit from a "void" caused by the delamination and then deposit on the surface above the crack. Draining a pool will promote the loss of bonding of new plaster to old plaster.
The best way to remove is to get some wet & dry sandpaper (either 80 or 100 grit) and simply sand off those raised calcium carbonate bumps, or just simply break them off the surface with a chisel type tool. Unfortunate, after removing them, many nodules will return or form again. When the pool water warms up, jump in the pool and remove the nodules, use a small pointed "pick" to open up the crack or hole, and then push some epoxy cement into the crack or hole to plug it up. That will stop the reformation of nodules.
Do not make the pool water aggressive or use acid on the nodules as it will also etch and aged the surface around the nodules and make it rough permanently. Also, let's understand that this problem has nothing to do with the water balance of the pool water. The nodules will form whether the calcium, TA, or pH level is low or high. You can't stop it unless you plug the crack or hole.
Incidently, calcium nodules are not the same thing as white spotting, soft spots, or spot etching, which are all the same plaster defect issue.