[prow]

that looks to me like the early stages of lymphocystis. i am pretty sure that is what it is. if so no known cure. it looks like you might have got it early enough to do a little surgery and cut out the infected area.
if you choose to do this, do it now before secondary infections take hold and/or before the affected becomes ulcerative and invades underlining tissue
after a QT with braodspectrum antibodies will be needed.