You could think about upgrading your XMS server to the latest version which is 10.6.
Before doing that please read the System requirements and compatibility information that can be found here
In fact even as today lot of customer still use ESXi 5.5 which is not EOL until 9/19/2018 (more than 1 year).
But supported version of VMware for XMS 10.6 is at least ESXi 6.0
Your XMS 10.6 should work with ESXi 5.5 but even if it does it will not be supported by Citrix.