well ok, here's the thing - STF and HistogramTransformation are both part of the same module - IntensityTransformations - and as such can 'talk' to one another by dragging the triangle from one to the other. IIRC if you try to drag the output of one module to another, incompatible module, you get that X to indicate that the two modules can't talk to one another. for instance if you were to try to drag HistogramTransformation's triangle to StarMask, i think you should see the X.
i don't know under what circumstances HT would not accept the output from STF. what happens if you reset the STF instance and just apply that to HT? do you still get the X?
i don't think you should need the checkmark active, as that's a separate function (causes HT to 'track', that is, switch automatically to the active view as you activate different windows.)
rob