Sounds like mealy bugs or soft scale. You need to get rid of them and make sure any of your other plants aren't infested. I don't use insecticides and rely on strong spraying with a hose (often) and carefully wiping the plants with cotton balls dipped in very diluted rubbing alcohol. There's plenty of insecticides if you decide to go that route. I think it's still too cold in your zone to keep a mandevilla outside. When it's warm enough to go out slowly acclimate it to full sun.