So, here are my 5e stats for this mycological horror. It's not actually a monster as such, but a pretty vicious terrain feature. However, unless the group are pretty low level, they shouldn't be too nasty....though "shouldn't" is a very relative word in D&D *evil grin*
Zygom patches typically covers an area of 2d4 contiguous squares. If anyone enters their space (which is difficult terrain), they must make a D.C. 13 Dexterity save or break several fungi. If this happens, they are restrained by the gluey sap until they make a D.C. 18 Strength check. They must also make a D.C. 16 Constitution saving throw to avoid becoming infected by the Zygom spores that lace the sap.
If they fail this save, they are infested by its aggressive pathogens and immediately become highly protective of the colony, only desiring to stay and defend the colony, and violently opposing any attempts to move them or to harm it. Every 1d4 hours they must repeat the Constitution save, failure indicating they take 16 (3d10) Necrotic damage, their total hit points being reduced by the same amount. If slain by this damage, they immediately rot, and a new Zygom colony is born.
If the victim makes three successful saves in a row, they fight off the infection, and all symptoms fade. Any lost total hit points return slowly, at the rate of 16 (3d10) per long rest.
A Neutralise Poison spell allows the target to save against the spores immediately with advantage. A Heal spell, Greater Restoration or similar completely remove the infection, the former also restoring all lost total hit points.
A patch (5' square) of Zygoms have 25 hit points, AC 6, and are immune to psychic damage. They have no intelligence, being in most respects normal fungi (albeit fungi from a remote world). As such they are immune to any attacks that rely on fear, or a functioning intellect, and are rather passive participants in any attempts to bluff or intimidate them.