FWIW, IME FW exit (enough acrynoms?) isn't all that risky to use unless you have a lot of the FW's.
IME, the one thing that the FW exit does/may harm is starfish. When I used it, my serpant stars seemed to go into convulsions or something like it. They were very stressed for about a day, but then fully recovered. My tiny brittle stars were another story. I hadn't even seen any tiny stars for a long time, but when I dosed FW exit I saw their arms and body parts falling apart and coming out of everywhere. It took months for the population to recover. Nothing else showed any signs of stress whatsoever. I did dose at about 1.5 times the reccomended dose.
If you dose, I would suggest temporarily removing any stars. otherwise no worries.
I agree with what cindy said about the ID test being weather or not the FW's start multiplying like crazy. The problem is that once they become a plauge, it's a major risk to use the FW exit becuase of the toxins the FW's will release.
IMO (and this is debatable) I would dose with the FW exit now as a preventitive. If they are the non-photosynthetic ones, you won't have any problems without them. If they are acro eaters (it doesn't sound like it at all) the FW exit won't do anything. If they are the nusiance FW's (which it sounds like, and the pic looks like to me) you'll get the upper hand before they become a plauge and a risk to try to kill.
Just my $.02