Keep in mind that the engulfing attack of SLASH'EM can corrode objects in open inventory (including your alignment key). In Vanila, only worn armor can be damaged this way.
The clear jelly has the ability to conceal itself, so adventurers may not see it until they stumble into it.
The yellow jelly possesses a passive stun attack in addition to the acidic engulfing. Eating the corpse of a yellow jelly will cause hallucination in addition to acid damage. It is much like the Yellow Mold that is seen in vanilla NetHack as well as SLASH'EM.
The orange jelly possesses a passive sleep attack in addition to the acidic engulfing. Avoid attacking them in melee if you do not possess sleep resistance.
Rancid Jellies possess the same basic acidic passive and engulf attack as their weaker brethren, with no other distinct ability. They are, however, the most powerful of all the jellies.