What is a systemic pesticide for scale on indoor plants?

The most powerful of the systemic pesticides doesn't need to be used against scale on indoor plants. Entry level organic pesticides such as horticultural oil or insecticidal soap get the job done, slowly but surely and safely. The oil may need a summer and two winter applications. It's particularly effective against the egg stage in the scale life cycle.

Insecticidal soap may be home made or store bought. Basically, it's just dish washing [not dishwasher] soap and water. It's particularly effective against the soft bodied crawler stage of the scale life cycle.

In coming up with a battle plan, it's important to identify the particular plant and the particular scale. In fact, there are many different kinds of scale. Generally, they ultimately respond to the above mentioned treatments. But there are exceptions. For example, insecticidal soap tends not to be effective against Japanese, lecanium, oak kermes, obscure, tuliptree or wax scales.

Also regarding a battle plan, it generally isn't advisable to use pesticides inside. The problem lies in their residual nature. Specifically, different pesticides last different amounts of times after their application in the environment. But horticultural oil and insecticidal soap aren't long lasting. That's why they tend to need a couple of applications to do the job.