Hi into 3rd year of owning this 4 year old fiberglass pool and start seeing this greenish long straight stain at the deepest end. Can’t brush them off so not algae. Water chemistry is normal. But this stain is strange in that it cuts clear on the edge ,see 1st picture . I am very concerned if it is not really stain but something going on beneath the pool or underground water leaking. Any advice is appreciated on how to diagnose this. I am in northern VA.
