Thank you for contacting Ask An Expert.
Unfortunately you have an infestation of a scale insect on the plant pictured. I am attaching a link to give you more information.
What I would do immediately is isolated the infected plant. We can have totally healthy plants in our house for years and then bring home a new treasure only to find that the new plant brought an insect pest with them. When I buy a new plant, I inspect it carefully (looking under the leaves is a good idea). I then try to put the new plant off by itself, perhaps in another room, and watch it to make sure it's healthy. This is good practice especially if you have plants already at home.
Scale is a common insect pest of houseplants (and outdoor plants too). Early detection is important for saving the plant and preventing the insect from spreading.
The insect weakens the plant and can kill it if the infestation is bad enough.The sticky substance you see is called Honeydew. It is the insect's excrement and has concentrated sugars in it, hence it is sticky. Scale insects suck the plant juices.
If the plant has a lot of scale on many leaves and it is not one you feel you have to keep, I would throw it out. If you want to save the plant follow the instructions in the link (I squish the ones I find or swab the insects with alcohol), I have also used a systemic insecticide. If you choose to use a systemic insecticide use one that is for houseplants and says it kills scale on the label. Always follow the label's instructions exactly.
It can be a tough insect to eliminate so if you keep and treat your plant you will probably have to repeat this process.
I hope I have helped you. I have had this problem many times. It is not a death sentence but it does require observation and diligence. Good luck.