I'm guessing the safe one is something like barley extract or barley
pellets. I've had some luck with that, but mostly as a preventitive,
not a remedy. I wouldn't go near the one with warnings, especially if
you have other plants that you don't want to kill off. I'm guessing
it's an algaecide, and those can be pretty indiscriminate.

You do really need to figure out what the problem is, and address
that. Nothing else is really going to make a long term difference.
Much of the time it comes down to too much sun in a shallow pond, too
much crud (aka nutrients) in the water either from fish overload or
insufficient water flow & filtering. Also, fertilizer getting into the
pond either intentionally for water plants or unintentionally, (run
off from nearby plants) can cause algae blooms.

I oversize my filter setup since it's a safe bet that my fish
population will increase dramatically every year. If you have an
inadequate filter and pump combination, that's probably your biggest
problem. What kind of setup do you have? By the way, if you use
something to kill the algae, all that dead algae is going to need to
be cleaned out or you'll have more food for the next wave. In my
opinion, the pump & filter are the most important part of all this. A
good pump and filter setup can compensate for too many fish and reduce
the amount of crud in the water. Conversely, if you have a small
number of fish in your pond (or even NO fish) but a poor filter
combination, you're very likely to have algae problems.

You can do water changes to get some of the nutrients out, but you
need to figure out why there's so much in the water to start with. I'd
start by making sure your pump and filter are suitable for the pond.

- dave