SP_WhoIsActive - RESOURCE_SEMAPHORE_QUERY_COMPILE
The sp_WhoIsActive documentation says:


  The [used_memory] column is also reported based on a number of 8 KB pages. The number a combination of both procedure cache memory and workspace memory grant.


However, I need to point out a few things in your question:


Queries that are waiting on compilation memory aren't shown in your screenshot
Those queries waiting for compilation memory are looking for a different kind of memory (gateways) - I'm not saying you don't have memory pressure, I'm just saying I probably wouldn't sort by memory_used to track the problem down
Your screenshot shows a serious blocking issue - queries have been waiting minutes (not seconds) because they're blocked by others


In your case, I'd start by troubleshooting the lead blocker. Don't worry about the queries waiting to compile - they haven't even started yet. In your scenario, the last thing you should be doing is starting even more queries, which are likely to just get blocked anyway and consume more workspace memory while they wait. Get the lead blocker out of the way instead.