When they die they melt away, if theyr're in bad shape they won't stay anchored to anything.
I'd say, especially if new to your tank, it may still be adjusting to the system. If it's foot is still holding firm to whatever rock it's on I wouldn't be too worried yet.
Another note. Mt RBTA's have endured me being lazy with husbandry at times and fluctuating water quality as well and they're doing just fine, just keep close tabs on it and keep up with your maintenance.