A spotted jelly is an immobile acidic monster, possessing only a passive attack. As with blue jellies, the damage of this attack is based on the monster's current level. Being acidic, its attack can also corrode your weapons and armor, and its corpse is acidic to eat.
Although it does not move, its passive acid attack can deal quite a bit of damage to unsuspecting players. Resistance to this attack is fairly hard to gain, being provided only by yellow dragon scales/mail, an alchemy smock, or self-polymorph. For this reason, these monsters are usually best left alone unless they are blocking something vital, which is fairly rare. If you need to kill one, find some rocks from somewhere and chuck them at it.
Be very careful with valuable pets around these monsters, since they tend to blindly attack them and die. Mind flayers, sustaining acid damage for each individual flay, are particularly vulnerable.