We have success.
Now looking at the edges of the ulceration, it does appear that the mass has sloughed from the head without the influence of any external factors (ie trauma, tearing, etc). And this is something that can arise with these types of masses. This happens when the mass becomes too big for the blood supply supporting its growth. And when there is too little blood, we can see the tissue become devitalized and it then will slough.
In this case, the mass's sloughing is neither good or bad in itself. In some cases, the mass may not regrow (though they can have scarring) but often they do. Where this situation can become concerning (or bad) is when we have open lesion (red/raw/etc) that is at risk of infection by secondary agents (often Aeromonas).
In this situation, you might consider supporting this fish with Melafix to aid healing. You can also use a water probiotic like Koizyme to discourage Aeromonas population within the water. As well, supportive measures like maintaining high water quality, keeping water temperatures in the high 70's to low 80's, and even using a low dose of aquarium salt (ie 0.1%) to encourage healing would be beneficial here. While doing this, you do want to keep a close eye for any cotton/fuzzy material or inflammation/red streaking around this ulcer, as these can be hints of bacterial infection and a red flag that antibiotic cover (ie kanamycin/doxycycline) would then also be indicated.
Overall, this is not an uncommon result of a lymphocystis mass outgrowing its blood supply. As long as the resultant lesion is not open or raw, I would advise supportive care to encourage healing. But if you do see any raw areas on the lesion or any hints of bacteria preying upon it, then we'd want to consider starting antibiotics against opportunistic bacterial agents.
All the best,