Psionic, crystal-eating aberrations with four HD. The parallels with rust monsters are obvious, except folugubs just kind of fail to perform a similar role.

At low level, even a fully psionic party will probably own more metal items than crystal ones. This is especially true at low levels, when a dumb CR 2 monster like this is most relevant. The only way in which I could see one of those actually harm the party is by means of something like a trap set to drop fogulubs on the party psion, or something, which would be effective but hardly fun.

Stat-wise, folugubs are unimpressive. +6 constitution and dexterity, +2 wisdom, -8 intelligence, no thumbs, a weak 2d4 natural bite, scent, and 'Liquify Crystal', an ability that lets the folugub waste its one attack a turn on liquifying a piece of crystal loot. Good job, that's totally as useful as a regular 4th-level PC.

If it wasn't obvious enough, folugubs are terrible monsters, terrible PCs, and do not deserve anything other than -0 LA.