If it’s the thing on the glass, it’s a nudibranch, maybe berghia but I doubt it is that simple. There are so many types, some harmless and some not so much. Clearer pic of the top will help.
Compare to Phestilla subodiosus, the montipora eating nudibranch. If you have montis in your tank then keep an eye out for more. They can grow to roughly 1/4 inch. The cerata on the individual pictured appear too few and spaced out to be a berghia.