Index is seeked and scanned - is there a risk for seek to fail after modification?
I would start by adding a clustered index to the table. Since the primary key is unique I would just update that from nonclustered to clustered. To do that you will have to drop and recreate the constraint so if you have foreign keys defined you will need to drop those constraints first then put them back once you have the primary key put back on.

A few references on clustered indexes

Clustered indexes vs heaps 

RID lookups

If all of your queries against the table include acatalid and acategid in the join or where clause then adding acatalid to the existing index may help.

However if the queries don't always include both columns in the where clause you may see a benefit in creating another index on acatalid. 

I say these changes may help as the statistics that SQL server has on the table could be generating an execution plan with the scan because it sees the cost of scanning through the table as less than doing a seek.