TL;DR: there's not enough chlorine, not enough of that is it a convenient form, and there's far too much water to turn an earthlike world into an acidic chlorine world any time soon. Unless your attack timescales are very, very long, you'd best find an alternative attack mechanism. The timescales involved also give enemies lots of time to find counters to the xenoforming gloop.
Problem one is getting enough chlorine. In the solar system in general and earth in particular, it is good 4 orders of magnitude less common than oxygen, for example. This strongly limits how chlorinated your final world will be.
Now, the easiest place to find a good supply of chlorine on earth is in the sea, so your chlorine-producing organisms will need to be photosynthetic algae. Better hope your terraforming organisms are super efficient, because they'll be competing with existing phytoplankton that are very well suited to their environment (which chlorine organisms will probably not be) and are present in enormous numbers and will compete for nutrients.
Take the next bit with a pinch of, aha, salt, because I Am Not An Alchemist. The H-O bond in water (which is broken during photosynthesis) has a bond energy of a mere 467kJ/mol, vs the Na-Cl bond in salt of 769kJ/mol. So on top of all the other problems you have, you'll need a photosynthesis process which is significantly more efficient than the terrestrial kind (or whatever other process is on the "garden worlds" you're trying to destroy).
Next, when grabbing the hydrogen from water or HCl the other bit, the waste, is gas phase and so is easily lost. In this case though you end up with sodium and potassium ions, which will presumably form sodium and potassium hydroxide in pretty short order, which is going to really put a dent in your hopes to turn the sea into acid as both those chemicals are strong bases.
The free chlorine your algae produce will react with water to form hydrochloric and hypochlorous acids, and given that earth has quite a lot of water you'll have a real problem keeping your liberated chlorine (which is heavier than air, and so will naturally pool at low points... eg. over the sea) from simply dissolving back into the ocean, and then react with whatever else is available (all that sodium and potassium from the sea salt has to go somewhere, right?). Surface saline water may turn into weak acid, which will be obviously quite bad for marine ecosystems, but there's a lot of sea to dilute that acid and a lot of highly basic metalbolic waste from your algae, such as sodium and potassium hydroxides which will neutralise that acid and turn straight back into salt.
The process will also liberate oxygen from sea water, I'm not sure if this is enough to offset the loss of oxygen production from phytoplankton, but you never know.
In chlorine poor areas (like the land) and places which are comfortably above the level of any free chlorine liberation (like most land) it isn't obvious what sort of effects you'll actually see. If you're hoping for rains of acid and choking green fog then I suspect you'll be disappointed. It'll be an ecological catastrophe and one that might ultimately render the planet uninhabitable, but it'll take a looooong time.
Also relevant: What conditions are necessary to support a vitriolic chlorine planet? (and a few other questions on chlorine planets) suggest that you need a pretty hostile acidic atmosphere in order to end up with free chlorine. You'll need to handwave some other process to trigger that... some kind of massive runaway greenhouse effect to get a venusian world that then might be further xenoterraformed? In any case, it won't be the chlorine that renders the world uninhabitable.