A 'slug' is any unit of mass inside the gallbladder. This mass unit could be anything from a cancerous or non-cancerous growth to a stone that requires imminent removal. To properly understand what a slug is and how you need to approach/treat it, though, it's vitally important to first of all undergo an ultrasound scan, as ordered by the doctor assigned to overseeing your treatment.
Detailed ultrasound scans on the gallbladder allow doctors to analyze what the unit of mass actually is, using the size and shape shown on the screen to help identify the object and track down the cause behind its presence. The gallbladder is effectively just a sac in the body that's used to store and concentrate bile that's produced in the liver, so any solid mass is likely to be a gallstone or unwanted sludge build-up, but could possibly be something more or less serious.
If the problem causing the slug is related to gallstones, it's likely that keyhole surgery will be required to remove the cause of the mass from within. Likewise, surgery may be needed to rectify a sludge build-up, although this can be rectified earlier on in its development by controlling one's diet in accordance with medical advice given by your health practitioner.
With any gallbladder slug, there are some common symptoms to look out for and be alert yourself of in order to avoid the need for surgery and limit any potential damage to your health. One of the main symptoms of a slug is abdominal pain and nausea, whilst acid reflux after eating fatty foods may also present itself in the case of gallstones. If not treated early enough, gallstones may lead to a bacterial infection of the bladder and/or severe inflammation of the gallbladder, causing jaundice and related skin discoloration as well as the basic symptoms listed previously.