I already looked in the "what is this bug thread" but this is very unusual... Ugly... And gives me the chills. The closet thing it looks like is planaria. But this is different... It's fast.... It goes from being 1cm and stretches to 3cm in a blink of an eye. It's brown. And it seems to attatched itself to the glass with a mouth kind of like a leech... And it can swim, it wiggles. Ill let the pictures do justice. I was able to catch it and put it in a spoon
You should absolutely be concerned. They can do a number on a shrimp and are dang near impossible to get rid of short of breaking down the tank and bleaching everything. Hope this was just a lone leech.