The "crusty" substance you refer to is in fact EPOXY. It is an important, albeit unsightly, seal on the flyback transformer. Per Larry Pina atop page 96 of his excellent Macintosh Repair and Upgrade Secrets book, broken epoxy seals result in ringing noises. This could result from a broken seal at the top of the flyback (the "top" is the location of the seal you have pointed out to us) or from a broken epoxy seal at the bottom of the flyback (which cannot be viewed until you desolder and remove the flyback). Epoxy can be repaired in most cases by applying new epoxy, but as Pina states, it's often best to just replace the flyback. In any event, you need not worry about replacing the flyback unless you hear unusual noises, or perhaps if you see wiggling on your screen or smell ozone.

I have take some photos in the past which show how my Mac128/512 and SE/30 flybacks look, with uncracked epoxy in good condition:

• Pre-Mac SE Analog Board: look to the left of

this photo.
Pre-Mac SE Analog Board: look to the upper left of this photo.
SE/30 Analog Board: look to the upper right of this photo.
SE/30 Analog Board: look to the upper right of this photo.