Unexpected sinusoidal wave at low voltage setting on oscilloscope?
Small waves like this on a scope are almost always from noise from another device. Make sure that the outlets are grounded properly. 200Hz is a strange frequency for noise to be appearing on, but it is too exact to not be a device of some kind. You should also check the lights in your room as well as other appliances.

50mV peak to peak is relatively small. If it isn't interfering with your other measurements it shouldn't be a problem.

You can also try shielding your power supplies, probe, or scope, or any other appliances with tin foil to reduce or eliminate noise.