This happened to me with a small tailspot blenny, I ended up getting two small extendable nets from amazon because I was scared of my plumbing ha. I'd position one the full length of the overflow and one maybe half so you maneuver easily. It took me 3 or 4 tries but was ultimately able to nab him by moving him up towards the top. If comfortable with removing the overflow, that's probably the easiest, just have something below to catch like a filter sock or net on the way down.
I puttied the area of the overflow he used to escape and haven't had any other fish go in.