The rules are clear:
1. The No. 1 reason that people do moolighting is because they are underpaid.
2. All employers started by moolighting, it is fair game to them. They will know about your moolighting before you bring your first dentist appointement as a reason for leaving the office.
3. You live by the sword, you shall die by the sword. Be ready to lose your job if your work suffers, most employers will leave you alone if you continue performing as usual. It is better than giving you a raise to keep you from moolighting. It's nothing personnal, it's strictly business.
I moolighted for quiete a while, quite Frankly, if I did not, I would not have survived.
It is easy to moolight doing design work, but when meetings and construction RFI's and punch lists, surveys, etc come in and need your immediate attention while your boss wants the job out by noon, it can get really tough, your vacation time will disapear very quickly. That's what makes most moolighters quit, like I did.
Like moltenmetal said here: Please price it right, and make it worthwhile. Remember, beware of those clients that tell you, do this for a low fee this time and I will have a lot of work for you. What they mean "they will have a lot more cheap work for you".