Re: Does someone know what this is?
Boy, do I not like the looks of that. If someone's had the neck off, "almost brand new and hardly used" is irrelevant. A pro repair would never (a) use a glue that looks like that, or (b) leave that much glue visible after the repair.
I'd take it to a good instrument repair shop and have it professionally inspected. If the neck's properly set (or reset), and the repair is stable, you're probably "OK to go" and keep enjoying the mandolin. There may be some cosmetic clean-up possible so you don't have that glop at the neck joint. What you won't know is, if further repairs are ever required, the joint can be unglued and redone.
Of course, it might be just residue from some superficial refinish or such, that accumulated at the neck joint. But it doesn't look like that. I'd get a pro's evaluation, if only for reassurance.
Gibsn: '54 F5 3pt F2 A-N Custm K1 m'cello
Natl Triolian Dobro mando
Victoria b-back Merrill alumnm b-back
Stradolin Vega banjolin
Sobell'dola Washburn b-back'dola
Eastmn: 615'dola 805 m'cello
Flatiron 3K OM