What is the difference between sqq and ff?

The variation appears to be accidental. Pick cites the same 1904 work by Hennecke elsewhere and uses ff, not sqq, in those citations:

Hennecke, Neutestamentliche Apokryphen, 1904, pp. 357 ff. and Handbuch, pp. 358 ff. (Pick, p. 1.)

"Petrusakten" in Hennecke, Neutestamentliche Apokryphen, 1904 383 ff.; in Handbuch zu den neutestamentlichen Apokryphen, 1904, 395 ff. (Ibid., p. 50.)

Schimmelpfeng-Hennecke in Hennecke, Neutestamentaliche Apokryphen, 1904, 459 ff.; Handbuch, 1904, 544 ff. (Ibid., p. 200.)

Raabe-Preuschen in Hennecke, Neutest. Apokryphen, 1904, 473 ff., and in Handbuch (1904) 563 ff. (Ibid., p. 222.)

Further, on page 359 of his 1904 Handbuch, Hennecke discusses the acts of Paul and Thecla, which is the same topic for which Pick has cited 359 sqq of that work. (Pick, pp. 8, 9.) The first full paragraph of that page begins, Ueber die Acta Pauli et Theclae, ... ("About the Acts of Paul and Thecla, ...")