This is actually an incredibly good point. WIth this line of thinking a sessile organism is just a non-locomotive organism that as evolved some mutation to attach to substrates.
This is actually the case for most “sessile” organisms, barnacles start life drifting through the ocean until they find an attachment point, same with coral. And clams and so on.
And clams maintain the ability to move around into adulthood.
For a single celled version of this, look at the stentor, it has a mutation to attach to substrate , but it only stays attached as long as it is finding food. If it can’t it detaches and goes somewhere else. WHen hunting it is actually very actively moving around despite being sttached to a substrate.
