..... isn't it likely to be stuck so fast that it will just stay there in normal use?
Not at all !!!! Most of the build-up is sequential layering of filamentous matts of living, dead, and dying microorganisms mixed with bacterial slimes --- if bacteria are a prominent component of the 'mix' in the massive 'colony' (calcyx). The chief contaminant of most 'fouled' diesel tanks is usually a fungal - Cladosporium Resinae or similar species of which most all are 'resin formers'.
Eventually when the underlayers of dead cells begins to decompose/putrefy and there isnt significant 'asphaltene', etc. (the cellular poop that these resin formers 'excrete') to hold/'glue' everything together .... then during a heavy seastate you have a fairly good chance of whole sections breaking loose and starting an overwhelming 'particle storm' to your Racors, etc.
Each typical small Racor only has a 'dirt capacity of approx. 35-50 grams (1 to 1-1/2 oz.!!!!) of 'crud' before they 'choke'.