If a college does not require me to give them my essay score, but I took the SAT with essay and my essay marks are bad, can I hide just the essay score?
No, your essay score will always be sent along with your multiple choice test scores. You can't choose to send one and not the other. From the College Board site:
"If you take the SAT with Essay, your essay scores will always be reported along with your other scores from that test day. Even though Score Choice allows you to choose which day’s scores you send to colleges, you can never send only some scores from a certain test day."
Although colleges will see your essay score, if they don't require the essay it probably won't be much of a factor in their assessment of your qualifications. I wouldn't be too concerned about it if your multiple-choice scores are solid.