Hello. I found a strange sluggish invertebrate in my tank today. I haven't added anything to my 29 gallon tank in months and it seemed to come out of nowhere

The size is about 3/4 inch in length and 1/4 inch in width. I shot a quick video if that will help identify it.

Please let me know if you can figure out what it is and if it should be considered harmful to my reef

Thanks