Cannot make mask on shape layer

Mylenium. Thanks for your interest

Screen grab 1

I create one word of text (“HELLO”) and put a white line beneath it. The line is made from a skinny rectangle that comes from a shape layer. The objective is to make both the word and line disappear mid-screen and, to do this, I created a mask, initially in the text layer. 

Screen grab 2

I use the pen tool to draw a mask round the text in the composition window. 

Screen grab 3

I mark a keyframe in the mask path of the text layer. I allow the playhead to advance a few frames. I move the mask down to the bottom of the composition window and the text disappears. Then I mark my second keyframe in the text layer mask path. 

Screen grab 4

Now I try to make the white line disappear and this is where it all falls apart. I choose the pen tool after selecting the shape layer I used to make the white line. 

Screen grab 5

I try to mask around the white line with the pen tool. The first thing that happens is After Effects begins making new shape layers (red arrow in the screen shot). Secondly the edges of this “mask” become visible - that’s to say they look just like the white line itself. This is (I think) because the stroke tool has not been disabled. Thirdly, the new shape layers begin turning into solid shapes, presumably because the fill has not been disabled (fill is not visible in screen shot 5 since it was from an earlier iteration). 

Screen grab 6

I know I can disable the fill and stroke tools but it doesn’t help make the white line disappear in the way I was intending. If the stroke, for instance, is turned off, the white line is not visible in the first place since the white line has been built using a broad stroke or, previously, I used fill to build it. Either way, disabling the stroke and fill is not achieving anything helpful. The final screen shot 6, shows where I ended up (confused!).