It depends on the size, tank size, other species and individual personality of the fish. I've seen them play nice and I've seen them exhibit aggression.
I've never seen anything bother a possum wrasse. I have one in my 240g. reef with all manner of larger wrasse and fish and he is completely ignored. They are very cryptic though, you will rarely see it.
Mystery wrasse are usually aggressive you should have added the possum first especially since its in a 60g. I'm not saying it will definitely be a problem but most likely, I've kept them together in a similar sized tank worked for about a little less than 2 years and then the mystery flipped the script. Is it a 60 gallon cube?
As Randy has written it's hit or miss with them, but they are in the same family as Sixlines which is why I will never keep one. IMO Flashers or the smaller species of Fairies make better tank mates.