@flareyflan I've use SP on submerged algae too - just sprinkle it on the surface of the water and it will sink to make contact with the algae. The algae will turn white/gray and then be gone completely in the next day or two. I have a couple of small (behind some marginal plants for example where the fish can't get to poke around) "dead" spots in my pond - areas where the circulation is low that tend to grow the green, bubbly, slimy kind of algae. I treat those once or twice a season now and it keeps those areas clear.
Having said that, there are several things to remember:
1. The algae is NOT a problem, in and of itself. It is another form of aquatic plant life that is doing what nature intends and clearing your water of things that would otherwise be harmful. It's a problem when it gets out of balance and you have TOO MUCH, but your goal should never be to rid your pond of algae. That would be a fish tank, not a pond. A heatlhy carpet of algae on your rocks and/or liner is a good sign.Too much algae is a clue to you about your water quality- correct that and the algae will get under control.
2. Any time you kill off algae you need to be aware that you are FEEDING MORE ALGAE GROWTH. The dead and dying organic matter is like candy for algae. So whenever possible you should be removing string algae by hand BEFORE you try to eliminate it. The more you get out of the pond before it dies, the less of a rebound effect you will create. Once you've removed the bulk of it by hand, then treat the "bed" of the algae - the spots where it adheres tightly to the rocks... you'll know it when you see it!
3. Over time, your pond WILL balance itself, if you allow it to. But the more chemicals, additives, products, etc you add, the longer that process will take. Keep your pond clear of organic debris, keep your fish load low, feed your fish lightly (if at all - goldfish do fine in a garden pond left to their own devices) and you will see a remarkable change in your pond, seemingly overnight. There are lots of posters here who have experienced it themselves and will tell you - patience dear ponder!
As for the discoloration on your waterfall rocks - this is the one spot where you can really use SP effectively. With the waterfall off and the rocks still wet (spray them with the hose if you need to) sprinkle the SP on the areas that are discolored. Allow the powder to dry completely - the longer the better, even overnight if you can. SP is a bleaching agent - the same stuff that is in OxyClean for laundry - so it will remove that discoloration. HOWEVER - it is natural in a pond and it WILL come back. So if you are determined to have clean waterfall rocks, you may be doing a lot of cleaning!
Also, as your water clears you may find that the color of the rocks changes and becomes less of a distraction to you. So honestly I would leave the waterfall alone until you are happy with your water conditions.