If your pond doesn't already have string algae, chances are that any you put in there will die off.
The spores for algae are pretty much omni present. If the conditions are right, algae will grow. It's not the end of the world.
I tried adding string algae to my pond and nothing grew from it. Either the fish ate it or conditions were not right for it to grow.
I get string algae on my water fall, but not in the pond. I periodically pull it off the falls and toss it in the pond. The fish go nuts eating it up. They swarm around my legs waiting for the next piece to be tossed in the water. The next day it seems to be gone.
As far as critters in the pond. The ones that gross me out the most are the blood worms. ECK! I use to get in the pond up to my waist until I cleaned the pumps the first time and found all these little red worms squirming everywhere!!! AHHHHHH! ever since then I only get in the water up to my knees. LOL