It looks like 'Honeydew'. Honeydew is a waste substance produced from sap sucking insects like aphids and scale. Without a picture of the insect, I can't tell you exactly what insect it is.
The best and safest way to get rid of most insects on houseplants is soapy water. The homemade insect soap spray I like to use is to mix several drops of Dawn dish liquid (you can use a different kind of dish soap but Dawn is the best) with distilled water in a spray bottle. Spray the entire plant very thoroughly. Make sure to get all stems and both the upper side and under side of the leaves. Repeat spraying the plant with the soapy water every 3-4 days until there are no more signs of insects or honeydew. This usually takes about 2 weeks.
** Scale are very obvious black or brown hard shelled insects that attach themselves to a specific spot on a leaf or stem. The only way to remove them is picking them off with a Q-tip dipped in alcohol. They produce lots of honeydew but from the pictures you posted and your description, they are unlikely the problem. I wanted to include them in my answer just in case.