You mean the bottom cover I hope, .....the back cover would be indicating the monitor cover.
merely taking the bottom cover off you wont be reaching the spill (most of it) as you indicated you spilled it near the fan
dont dare turn it on, water / liquids migrate.
Turning it back on is BAD, damage A can lead to B leads to C etc.
Also water migrating around, especially while on, can wreck havoc worse than initial damage.
Nobody here can speculate on damage on something they cannot see.
Having repaired laptops in the past, even IF you got it completely dry, residue from water (iron particles etc), and wet spots under fine contacts is nearly impossible to "dry out", its just doesnt work that way.
Coffee even worse on the residue end which is nearly as bad as liquid shorting.
Lots of people try to "dry it out", most to learn little was solved in doing same, and sometimes further damage occurred.
Taking it apart, and the hidden spots will REMAIN wet for some time, even IF you dry to "dry it out"
Make appt. with Apple store fore repair / diagnostic.
dont turn it on. Sorry about the spill 😟