My rock nem moved just a bit from its original spot and this shows up. Wondered if it might be a little one? It's a bright orange/ red like the bigger one.
Making sure that I'm looking at the same thing...middle of page and red/orange bubble? If so, that probably just some red valonia or red bubble algae. Baby rock nems look identical to their parents with oral disk and tentacles, but just much smaller. Hope that helps.