Interslice tension is a common occurrence in slope stability models with cohesive materials in the upper part of the slope. What this implies is the existence of tension cracks. So to get rid of the tension you need to add a tension crack layer.
See page 222 of Duncan and Wright’s Soil Strength and Slope Stability text for more information. The reference is provided below:
Duncan, J.M. and Wright, S.G. (2005). Soil Strength and Slope Stability, John Wiley and Sons Inc.
Alternatively, rather than add a tension crack manually, for the strength models listed below you can define a tensile strength:

MohrCoulomb

Undrained (Phi = 0)

Anisotropic Strength

Anisotropic Function

Vertical Stress Ratio

Discrete Function

Anisotropic Linear

SHANSEP
The Tensile Strength option can be very useful for models which exhibit tensile forces between slices or on the base of slices. Large tensile forces in limit equilibrium slope stability analysis usually lead to incorrect solutions and/or numerical problems, and should generally be avoided.
The Tensile Strength option can be used to automatically eliminate tensile forces by adjusting the local factor of safety on a slice, so that the effective normal stress is zero on the base of the slice. If tensile forces exceed the tensile strength, a tension crack will automatically be created for the slip surface.