Criminal case dismissed and expunged. Employer applying for green card do I need to inform my employer regarding my case.

Criminal law and immigration laws are vastly different. For instance, for the purposes of your criminal history for any prospective employment you have no criminal record. However, for the adjustment of status the USCIS requires disclosure of any criminal history and its disposition even expunged. Please discuss your case particulars with a licensed immigration attorney for a better understanding how to proceed.

What to tell your employer and what to tell USCIS are two spearate issues. What you need to tell employer is governed by state law, specifically the Illinois Human Rights Act. In IL, employers are not allowed to hold expunged records against you when making employment decisions. You are also allowed to truthfully state that you have no convictions if asked. Any time you are petitioning USCIS for any immigration benefit (like you will be doing with the I-485), you have the burden of showing your "good moral character" and you will need to answer questions even about expunged criminal records. However, you can get your own attorney rather than doing this through your employer's counsel, and you do not need to tell the employer what employer has no right to know. Generally, expunging or sealing criminal records is not a good idea for non-citizens. either process makes the record of the crime hard to access, thus hard to show USCIS that crime was not one that should make you inadmissible/deportable.

Not the employer but USCIS yes on the I-485 form.

This question is better suited to an immigration attorney.

The matter would need to be disclosed at the I-485 stage.