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I used a greeenscreen to cut myself out of a video. I want to use that alpha/ mask on another layer. How can I link the mask from greenscreen layer to another layer?
If you have a Mask applied to one layer, you can copy and paste that to another; however, I think you're referring to using the layer transparancy (or Alpha) of one layer to another.
A Layer Track Matte is one approach. School of Motion offers a great quick explantion in their blog here.
You'd put the Fill Layer (they layer you want to see) behind the Matte Layer (the layer you want use the transparency from) and then set the Track Matte pop-up menu to Alpha "<Name of Front Layer>".
Another approach is to use the Set Matte effect (Effects > Channel > Set Matte).
Pre-compose the keyed result, use the pre-comp as often as you like in other comps, including as a track matte for other layers. You may want to read the online help and find a good tutorial on some of these basics.