watering with algae in rain barrel

The city i live in hardly ever gets rain but a few weeks ago we got a few rainy days so I quickly setup a makeshift rain barrel using an old trashcan. It filled up and ive been using it to water my drought intolerant plants. The water in the trash can now has this vague green mist all throughout it. I figured if anything this would give the plants extra nutrients. Is that thinking right? should I be doing this?

we have a similar situation. Just got a couple of the blue sort of barrel, of which have no idea what was in them previously. But i would imagine that the algae would not harm a plant.

I catch most of my irrigation water in barrels or above ground cheapo pools. Can't afford a better system yet, and no piped in water. Each barrel or pool gets a starter culture of water plants (duckweed, azolla, etc) and mosquito fish. The fish nibble plants and algae, and keep down water-reproducing insects, and fertilizethe water with poops. I can harvest a good few handfuls of water plants each day for my chickens and ducks. This is probably less relevent in temperate areas than my own subtropic climate.

Water plants on the surface of a pool reduce evaporation loss. 

I think algae water is fine for irrigation, but a more complex ecosystem is better - a mini-aquaponics system. A more complex system should also reduce pathogen load. So my advice - add more life to your barrel!

On a diffrent note - three years ago I could buy 55 gallon barrels (plastic, food grade) for $5 each. The last one I bought cost $35 and I've seen them posted to craigslist for $100.