That's a tough one. Clado is resistant to Excel/H2O2, and enough of those or other chemicals to reliably kill it will probably kill the fissidens as well.
I did once manage to kill clado on java moss with Algaefix treatment in a bucket, for three days, combined with blackout. Algaefix won't harm plants, but it's not known as an effective clado killer either. So I don't know if this is a guaranteed method. And at the time I put the moss back in the tank, the clado wasn't visibly affected at all, I just took it on faith that it was dead.
Fortunately it was, but at that time there was no way for me to actually verify that it was killed. It took a week for it to start decomposing. And with heavy H2O2/Excel treatment, it still might take three days to see effect.
So you're in one heck of a situation. Your choices:
1) Risk killing the plants to get rid of the clado
2) Risk a milder treatment and tank infestation
3) Or throw the RAOK out
If you can set up a separate quarantine tank that can be heavily treated, option #2 becomes most attractive; you can toy with the clado at your leisure, trying a variety of treatments and strengths, and still growing the plants in the meantime.