Masking issues (visible Mask)

I’ve never cared enough to look it up or try to fix it before but i think it’s time for me to figure it out.

When i used a mask. For example:

myClip_mc.mask = someMask_mc

I can still see my mask (barely) but it’s there and it’s annoying. It does it’s job of masking the right stuff but i can clearly see the outline of my mask. It kinda has a stripe pattern.

Also, it only happens if i set my monitor setting to 16bit colors.It’s fine in 32 bits.

Would someone tell me how to get rid of it or explain why it happens?