Your Yucca is suffering from lack of light, as Gene has pointed out. It is also in a pot that is too large and it appears to have a mealybug or scale infestation.
Move it to a location right in front of a sunny window. Allow the soil to dry one-third deep into the pot before adding just enough water so that it reaches that level of dryness again in about a week.
Mix a solution of 5 parts water, 1 part alcohol and a squirt of liquid soap. Then, thoroughly spray ALL leaf and stem surfaces until they are dripping wet. Be sure the get the solution down into the crevices where the leaves attach to the main stem.
