It's most likely totally fine.
Your best indication is to smell it. If it's not okay it will smell almost alcoholic....and 1 month is no problem if you used meringue powder and not fresh egg whites.
Storing it in the fridge may well mean you will have to add a little more icing sugar to it when you mix it up. The fridge changes the consistency as it sits and it's often a little thin when you go to re-beat it. 
It's most likely totally fine.
Your best indication is to smell it. If it's not okay it will smell almost alcoholic....and 1 month is no problem if you used meringue powder and not fresh egg whites.
Storing it in the fridge may well mean you will have to add a little more icing sugar to it when you mix it up. The fridge changes the consistency as it sits and it's often a little thin when you go to re-beat it.
Hi Antonia74, may I ask how long it is shelf safe if you used Meringue Powder and not egg whites?
Thank you!
Depending on how you store it, I make mine using meringue powder and it's fine outside of the fridge for weeks (say 4-6)....though I usually use mine up again within 2-3 weeks for another project. In the fridge, I find it gets thin and watery....I don't recommend it. I tried it in the very beginning and I found it ruined my deep/dark coloured royal icing. The freezer would be my second choice.
Your storage of it is the main factor though. It must be air-tight, completely sealed, sterile container, stored in a cool area, out of direct sunlight.
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