Sounds like watershield. The coating on the stems is a clue. A jelly like substance develops as the plant grows; this unfortunately makes the plant more resilient to contact herbicides. your description of the leaf shape and the red color to the underside fit watershield. Catch it as fast as you can. It spreads similarly to floating heart (Nymphoides peltata) by sending out runners on the bottom that develop quickly into new plants. Greg Grimes has a great recommendation. Hit it both ways, systemic (glyphosate) and contact (2,4-D).