Correct you would need to remove it from SANHQ and re-add it in via the Management Well Known address.  I don't know why you first had to go to the member Mgmt port vs. being able to use the IP alias for Group Management.   That's all the Management IP address is, just like the ISCSI discovery IP address.  It's an alias riding on top of a physical port IP address.   So if you can reach the member IP address you should have no issue reaching the alias.     Possibly something on your network, or the ARP table wasn't updated.  
TLDR: Yesterday I've updated SanHQ from 2.2 to 3.0.1 in preparation to running the upgrade path to FW 7.x. As of yesterday I'm getting the following warning: [ID: 1.6] The group eql-grp01 was added using non-management well known address (MWKA) address. Remove and add the group using the Group management well known address 172.16.0.9. The group eql-grp01 contains a single EQL (eql01 / 172.16.3.15) and has a dedicated management port. The range is a /16, as such both the group management IP and the EQL management IP are in the same (unrouted) network. I'm able to reach the eql01 webinterface on 172.16.3.15 without problems for years. Trying to reach the webinterface on 172.16.0.9 has always given problems (timeout, ping does work however) untill I've requested the webinterface on 172.16.3.15, after that I can reach the webinterface w/o problems on 172.16.0.9. Up untill now I didn't bother to look into it but as I will be addding another member to the group in a couple of weeks and SanHQ is 'nagging' about not using the well known management IP I tought I might give it a try here, perhaps this is known behaviour?  Edit: Adjusted the subject as the non-MWKA does work but the MWKA does (subject sugested otherwise)
Webinterface not working on management well known address