How about this fold?

***** Hand History for Game 1711188058 *****
$50 NL Hold'em - Wednesday, March 09, 19:10:01 EDT 2005
Table Table 37138 (Real Money)
Seat 3 is the button
Total number of players : 10
Seat 1: RakinIT_IN ( $45 )
Seat 2: SF_Horn ( $58.12 )
Seat 3: SlappYou ( $50.8 )
Seat 4: Stobe2k ( $49.6 )
Seat 5: xibustux ( $51.3 )
Seat 6: kimotang ( $44.8 )
Seat 7: JakeRanger ( $56.52 )
Seat 8: Nikifolds ( $106.3 )
Seat 9: FlyingNite ( $46.5 )
Seat 10: acalld ( $47.78 )
Stobe2k posts small blind [$0.25].
xibustux posts big blind [$0.5].
** Dealing down cards **
Dealt to SlappYou [ 6c 6h ]
kimotang folds.
JakeRanger folds.
FlyingNite calls [$0.5].
acalld folds.
RakinIT_IN folds.
SF_Horn folds.
SlappYou calls [$0.5].
Stobe2k folds.
xibustux checks.
** Dealing Flop ** [ 6d, 3h, 7h ]
xibustux checks.
FlyingNite bets [$2].
SlappYou raises [$4].
xibustux folds.
>You have options at Table 36598 Table!.
FlyingNite calls [$2].
** Dealing Turn ** [ 5s ]
FlyingNite checks.
SlappYou bets [$8].
FlyingNite raises [$16].
SlappYou calls [$8].
** Dealing River ** [ Jh ]
FlyingNite is all-In [$26]
SlappYou folds.

Too many hearts, and too many to a straight, it looked like he hit his straight, maybe even backed into a flush. I really felt like pushing here, but something about his betting seemed very strong.

Good fold or monsters under bed?

I'd say it was a solid fold.

bet the flop harder

with a dangerous straight/flush board, reraise him to 6 or 8 on the flop. make sure he's not on a draw.