Reef Safe is a relative term and often refers to whether a fish will eat shrimp, or coral, or it's tankmates.
To get the right answers, we need to ask the right questions:
1- What does an Emperor Angelfish eat

2- Is there any of that food in the tank

3- If there is little of that food, what will it turn to eating as a replacement

4- If there is lots of that food, will it be enough
I assume that your tank is fairly new, probably less than a year. In
that case we can assume there is little or no Emperor Angelfish food
growing in it, so questions #1 and #3 are the right questions.
Here is a pic from a tank for which the answer to #4 was "Yes" because the tank was 5 years old with plenty of algae, sponge and other Queen Angelfish food growing in it. This pic was taken when the Queen had been there for 2 years and had doubled in size.
