There's a couple of comments on XIVDB claiming that the key is foggy weather. I can't vouch for it myself since fog hasn't felt like rolling out in all the time I've been spending in the Sylphlands. ><

http://xivdb.com/?item/5022/Mushroom-Crab

xivtourist.com has the same info with a screencap of the actual catch, also saying fog is required:

http://www.xivtourist.com/2013/09/ff...room-crab.html