there is so much weird :pooh: that happens in our tanks that we cannot explain: fish behavior, coral behavior, little critters we cannot see doing bizarre things, etc. Somethings just are, and there is little we can do to figure them out. I like to look at my pretty tank and not try to understand everything that happens...or I would lose my mind (what is left).
Since your tank is not full of livestock, just ignore it and see if it goes away on its own. If it keeps coming back, then worry about it
-Doc