JSFiddle - React, Tailwind, and code Playground

HTML

<pre></pre>

JavaScript

f=i=>(I=i.split`
`.map(e=>e.split``),c=(i[m='match'](/C/g)||[])[l='length'],a=(x,y,z)=>{if(I[x][y]!=z)return;I[x][y]=0;x>0&&a(x-1,y,z);x<I[l]-1&&a(x+1,y,z);y>0&&a(x,y-1,z);y<I[0][l]-1&&a(x,y+1,z)},r=1,![...'CDEM'].some(k=>{if((i[m](eval(`/${k}/g`))||[])[l]!=c)return 1;I.some((w,x)=>(g=-1<(y=w.indexOf(k)),g&&a(x,y,k),g));})&&!I.map(e=>e.join``).join``[m](/[CDEM]/))

log = () => document.querySelector("pre").textContent += Array.prototype.join.call(arguments, "") + "\n";
test = v => {log("input:\n", v);log("output: ", f(v));log();};

test(``)
test(`DDDDDDDDDDDDMCCCCCCCCCCC
DEEEEEEEEEEDMMMMMMMCCCCC
DEEEEEEEEEEDMMMCCCCCCCCC
DEEEEEEEEEEDMMMMMMMMCCCC
DDDDDDDDDDDDMMMMMMMMMMMC`)
test(`DEMC`)
test(`DD
EE
MC
MC`)
test(`EEDDMMMCCC
EEEDDDMMCC`)

test(`EDM`)
test(`EDMCCMDE`)
test(`DDDDDDDDDDDDMCCCCCCCCCCC
DEEEEEEEEEEDMMMMMMMCCCCC
DEEEEEEEEEEMDMMCCCCCCCCC
DEEEEEEEEEEDMMMMMMMMCCCC
DDDDDDDDDDDDMMMMMMMMMMMC`)
test(`DDMMEECC
DMMEECCC`)