Make two circles and sit them on top of each other.
Add an adjustment layer over them.
Apply a blur effect to the adjustment layer and crank it up/
Apply the levels effect to the adjustment layer and - on the alpha levels (not the main levels) crush the blacks and whites until the circle looks solid and unblurred.
Now, animate one circle moving out from behind the other.
That may not be exactly what you're after, but it might be a good start.
Michael has a point, though this still leaves the problem that you will have to learn the finer points of animation and timing.... So whether you use masks or the old "blob blur" trick is realyl secondary. Both can look iffy, if done the wrong way...
I ended up using the CC Smear effect, so whilst one pertrudes the other is nested in the end of said protusion.
Then I animated the size and position of the nested circle (whilst protuding that one as well), so it appears to blob out of the end then the protusions retract from both circles, whilst putting an inertia bounce on the position of the 'End' position of the protusion. So it appears to wobble when it is fully retracted.
The problem with this is...The occasional glitch (when rendered) with CC Smear.
Is there a reason for this?