Hi Doris: Here is a question and answer I found regarding this: Here it is:
I collect water from my roof and store it in open dustbins for later use in my veg patch. Over time, it grows green algae. Does this make the water better for the veg (ie, the algae rot to give nutrients to the plants) or worse because some algae are poisonous?
It will have a nutritional value, but it's probably not very high. I wouldn't use this water on seedlings as you may get "damp off" (a number of fungal diseases that cause seedlings to die), often caused by non-sterile conditions. I can't imagine there is any problem with using the water straight on to vegetables in the ground. I'd steer clear of watering directly on to lettuce and other raw vegetables, and concentrate on watering the roots (which is better practice in general). If your soil and plants are healthy then this is like a weak, unspecified feed.
If you don't want the algae, I suggest emptying the dustbin and scrubbing it with boiling water. Then cover the bin with a lid, as algae needs sunlight. You could also put charcoal in the bottom to sweeten the water. A pair of tights over the end of the drainpipe will filter out any debris that will add to the buildup.
Here is another site that has a bunch of information:)
I didn't see a thing about golden algae.