A patient complains of an increasing new growth in the left sublingual area, which he first noticed 2 months ago. Some time after that he noticed this new growth to secrete large amount of clear viscous fluid, which was colorless and tasteless; initially this caused the new growth to become smaller but subsequently it started to grow again. Objectively: there is a rounded new growth sized 3x3 cm in the left sublingual area, its consistency is soft and elastic, there is no pain. The mucosa above is thinned out, semitransparent, bluish in shade. What is the most probable diagnosis?