I also had mesenterial filaments on my acropora. I would see mesenterial filaments on some acropora and not others. Some would brown out while others would recede at the tips. Only two things I changed in my tank was increase export (larger water changes) and added Kalkwasser. After I added Kalkwasser, I had a cyano outbreak maybe 4 - 6 weeks after and I had to cut back the Kalk to prevent my DKH from rising too high, I noticed the moment my PH dropped below 8.0 (from cutting back the Kalkwasser) I started to see the receding tips again. My tricolor validata was always the first to show symptoms. I bumped my dosage back up and basically said I don't care how high my ALK goes. My ALK went from 8.3 to 9.8, over the course of a week and there was no STN, or RTN and my corals tips continued to heal and this issue has not come back since I recovered from the Cyano outbreak. I have seen this kind of post multiple time's, and everyone seems to point to a DKH swing, or low nutrients and I don't think that's the cause. I have read multiple threads like this in search of an answer myself and I didn't find a resolution until my PH remained above 8.0 at all times. I have been wondering if it's something in my case bacteria related that just so happens to thrive or create an issue at night when the PH drops. This is also the first tank I have ever experienced this in, which is why I am wondering if its something bacteria related. I also switched from Brightwell salt early on to Aquaforest thinking maybe this was a problem, as my Brightwell parameters always seemed to mix lower than the bag states, but I still had this issue. I have never done an ICP, and at this point I only dose MG on top of my Kalkwasser at night. I would recommend a PH probe and at least understand your PH high and low point as it will be lowest at night. Just my two cents, throwing something out there that helped resolve this issue for me as you are just getting the typical answers, which brought me no resolution.