Is there any not-scary explanation for Finder's memory usage?
I wouldn't worry about any memory issue until there's pressure or a problem - specially if things are clean after a restart as you report in the comments. If Finder had a horrible bug, you and everyone else would be able to reproduce it taking 6 GB. In fact, when there’s a substantial leak, I expect 10 times as much allocation, runaway page file growth/swap and high memory pressure in short order.
Since you have none of those, I’d chalk this up to a transient condition or hard to trigger edge case at best.
The system is designed to use all the memory it can since memory access is faster than network or storage access. Self-managed systems can reduce the allocations once you have other needs. What your snapshot shows is nothing else needs RAM, so the system is using it all for other tasks.
I rarely see finder with high memory in the GB range so you might have something allocating this that you can control, but even so - your screen capture has no signs of pressure. You could dig into Finder's allocations or try removing Dropbox to see if there are leaks. Even if Finder is leaking, it's not causing any harm (perhaps yet). You can also log out and log back in. If you check on the RAM usage / pressure usage each hour for several hours, you can quickly determine if Finder indeed is leaking memory or if you just have a usage pattern where it allocated a lot of RAM and hasn't released it yet.
If you need to know, the heap command will let anyone diagnose their current Finder allocations:
$ heap Finder|more
Process: Finder [853]
Path: /System/Library/CoreServices/Finder.app/Contents/MacOS/Finder
Load Address: 0x1071b6000
Identifier: com.apple.finder
Version: 10.14.5 (1143.5.1)
Build Info: Finder_FE-1143005001000000~1
Code Type: X86-64
Parent Process: ??? [1]
Date/Time: 2019-05-21 06:36:12.965 -0500
Launch Time: 2019-05-19 09:56:56.140 -0500
OS Version: Mac OS X 10.14.5 (18F132)
Report Version: 7
Analysis Tool: /Applications/Xcode.app/Contents/Developer/usr/bin/heap
Analysis Tool Version: Xcode 10.2.1 (10E1001)
Physical footprint: 31.5M
Physical footprint (peak): 33.9M
----
Process 853: 4 zones
All zones: 90996 nodes malloced - Sizes: 300KB[1] 172KB[1] 132KB[2] 68KB[19] 60KB[1] 56KB[2] 48KB[2] 44KB[4] 40KB[1] 36KB[1] 30.5KB[1] 27KB[1] 24KB[1] 22.5KB[1] 22KB[4] 19.5KB[2] 18KB[1] 17KB[1] 16.5KB[2] 16KB[2] 14.5KB[5] 14KB[3] 13.5KB[3] 13KB[3] 12.5KB[4] 12KB[1] 11KB[1] 10.5KB[1] 10KB[5] 9.5KB[3] 9KB[4] 8.5KB[20] 8KB[13] 7.5KB[5] 7KB[6] 6.5KB[4] 6KB[4] 5.5KB[9] 5KB[16] 4.5KB[17] 4KB[43] 3.5KB[21] 3KB[62] 2.5KB[119] 2KB[144] 1.5KB[243] 1KB[63] 1008[3] 992[9] 976[3] 960[15] 944[4] 928[6] 912[5] 896[28] 880[24] 864[9] 848[6] 832[13] 816[9] 800[3] 784[15] 768[22] 752[11] 736[4] 720[7] 704[13] 688[14] 672[63] 656[17] 640[2] 624[32] 608[24] 592[25] 576[120] 560[10] 544[15] 528[219] 512[94] 496[23] 480[9] 464[59] 448[34] 432[85] 416[179] 400[59] 384[39] 368[91] 352[60] 336[615] 320[1231] 304[275] 288[196] 272[690] 256[364] 240[129] 224[1329] 208[253] 192[1413] 176[348] 160[354] 144[1809] 128[2413] 112[6448] 96[3460] 80[5112] 64[21054] 48[20747] 32[14405] 16[5997]
Found 1231 ObjC classes
Found 214 CFTypes
-----------------------------------------------------------------------
All zones: 90996 nodes (12977600 bytes)
COUNT BYTES AVG CLASS_NAME TYPE BINARY
===== ===== === ========== ==== ======
21548 5467536 253.7 non-object
16751 1044544 62.4 CFString ObjC CoreFoundation
5222 334208 64.0 CFDictionary ObjC CoreFoundation
5029 748640 148.9 CFDictionary (Value Storage) C CoreFoundation
4836 710864 147.0 CFDictionary (Key Storage) C CoreFoundation
3485 167280 48.0 NSMutableArray ObjC CoreFoundation
3421 134912 39.4 NSMutableArray (Storage) C CoreFoundation
1540 147808 96.0 NSURL ObjC CoreFoundation
1208 386560 320.0 _FileCache CFType CoreServicesInternal
1016 65024 64.0 TPropertyInfo C++ DesktopServicesPriv