I've got Flux Rx running right now in my IM 40g at 80mg/gal. That's 4x the dose for bryopsis, because I'm trying to knock out Valonia. So far, no ill effects on any corals/inverts/fish, but I also make sure to keep nitrates in the positive and am running dual air stones for the time being. Also adding PNS Probio intermittently. On Day 10 or so now, and the Valonia is greying out. The little bit of GHA I had has withered. Ulva is looking worse for wear. My macros are dying, except for codium, but I already trimmed and put aside frags in QT for replacement.
I would run it at 20mg/gal as indicated. Bryopsis is known to persist until the last cell is killed. What do you have in your tank that might be at risk?