Vermetids- a harmless hitchhiker and part of any tank- albeit a tank that perhaps may have been over-fed in its last life. The easy answer first: Nothing alive will be left behind, they're just shells. The hard part is getting them out of acrylic without scratching it; I'd use a vinegar/water solution and dab or spray it on until the shells are good and damp, and wait a day or so- then use a cloth and gently wipe them off.
If they aren't absolutely covering the tank (I've seen it, not pretty!), consider just leaving them. As you fill and cycle your tank, they may be largely unnoticed. Good luck and welcome to RC!
