Spidery ha is a rare Glagolitic sign, named in the Unicode Standard for the many-legged shape that distinguishes it, and it carries no numeral value. It writes a sound close to that of the ordinary letter for the velar fricative, and it belongs to the Croatian branch of the tradition rather than to the common inventory of the earliest manuscripts.
Letters of this kind are a reminder that Glagolitic was not one thing. The script had a long life in two distinct traditions, and the second of them, on the Adriatic coast, developed its own hand, its own repertoire and its own conventions over several centuries in which the script was extinct almost everywhere else. A sign attested only in that branch is a real letter of a real tradition and is not a letter of the alphabet the mission carried to Moravia.
Whether such a sign belongs in a list of the Glagolitic letters depends entirely on what the list is for. A catalogue of the tradition includes it. A reconstruction of the ninth century alphabet does not. The atlas follows the Unicode arrangement, which catalogues the tradition, and says so rather than presenting the result as the original.
Nothing about the sign is mysterious in the way the letter pe is. Its sound is known, its tradition is known, and the manuscripts that carry it can be placed and roughly dated. What is contestable is only its status: whether a form used in one branch of a script for part of its history counts as a letter of that script's alphabet. That is a question about definitions rather than about evidence, and different reference works answer it differently for good reasons.