Sorry for the tardy reply.
Again, nutrient stain is common in DWC. Algae generally won't hurt your plant, but hydroponics is normally sterile.
I suspect most of your shoot damage is pH related. The damage is generally something that happens a few days past.
I recommend following the instructions on the bottle for H2O2 application. Just be careful as the H2O2 designed for hydroponics is very strong (29%) and will burn your skin, and will raise your pH.
As an alternative, you may also try benefitial bacteria.