Is there any not-scary explanation for Finder's memory usage? OS X El Capitan 10.11.6. MacBook Pro (Retina, 13-inch, Early 2015).
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