I noticed this on my Forest Fire Digi. At first I ignored it because I have multiple wrasses that sleep in mucous cocoons at night and I thought perhaps it was one of those caught on the coral but it is still there after 2 weeks. The coral does not seem to be upset, good polyp extension, and there is no other corals close by that could be bothering it. Today I noticed it is also on my Purple Stylo (it was purple when I got it from previous owner but it turned green in my tank). Again no other corals nearby to bother it. Test is the most recent I have available. Picture and video were taken with orange clip-on filter on iPhone SE and zoomed in as close as I could get just after lights turned on this morning. Corals have been in my tank for nearly 6 months.