There could be several different reasons for that....
First of all, could you try Rebuild Topology link in the VEEAM UI? This causes SCOM to rebuild and re-initialize all the VMware objects.
Please wait 30 minutes or so for that to complete.
Second, you could also try restarting the System Center Health Service on the Collector server which monitors this host. (you can see in the VEEAM UI which Collector is monitoring which Cluster(s))
That will also cause SCOM to re-discover and re-initialize managed objects, including VMware hosts.
Again wait about 30 minutes for that to complete.
If those solutions don't work, there could be a more complex problem with vCenter, permissions, the host itself, etc...if you raise a support case we can dive in.