Computer science education has emerged as a recurring topic within the last years due to the growing relevance of skills in information technology. In particular, the discussions and initiatives focus on primary and secondary education, thereby stressing the significance of imparting the basic concepts of computer science and programming to children and adolescents early on. Programming itself is a demanding activity. Hence, in order to simplify it, many applications for beginners adopt a visual representation. Visual programming languages (VPL) use graphical components representing programmatic constructs, which are connected or placed in relation to each other to form programs. By focusing on semantic aspects of programming, while reducing the possibility of syntactical errors, visual approaches have been applied to compute science education at the secondary level. In this regard Scratch poses the state of the art environment enabling novices to create their own animations and programs. Scratch features a purely visual language based on color-coded Lego-like blocks representing operators, functions, or object attributes and has been intended for the use on traditional computers. While usually desktop computers and laptops have been employed to develop software for decades, the smartphone penetration among citizens of industrial countries provides new opportunities for education proposes in this regard. Owing to their pervasive nature, Internet capabilities, and sensors, portable devices can support learning anytime and anywhere. Mobile phones have become an essential part of teenagers’ and children’s everyday life, thus expanding the efforts of novice programming environments to smartphones and tablets represents the next logical step to foster computer science education. Pocket Code is a mobile integrated development environment (IDE) and interpreter developed for the VPL Catrobat targeting teenage users. Inspired by Scratch programs in the Pocket Code app are constructed via colored bricks while taking full advantage of the device sensors available. Achieving an appealing programming experience on mobile devices, however, is impeded by the small screen sizes and error-prone data entry methods. Especially considering formula manipulation, employing solely a visual representation can become cumbersome and confusing with the size of the formula. Although being a crucial part of programming itself, this issue has gained little attention in research so far. To avoid this issue, Pocket Code utilizes a hybrid textual/visual editor for constructing and adapting formulas. In order to evaluate the mechanics of the hybrid approach we carried out two independent usability assessments, namely a heuristic evaluation and a formal test. Regarding the former, a review of the formula editor interface of Pocket Code was conducted considering specific guidelines for mobile applications. Several reviewers were employed to identify possible issues violating the provided principles. Due to the heuristics used, the identified problems give a general picture of the usability of the formula editor besides the pure mechanics of creating or editing a formula. The empirical assessment was performed as a formal usability study comparing the purely visual formula composition and manipulation in Scratch to our textual/visual method in regard to efficiency, effectiveness, and perceived user satisfaction. While the test results indicate that the hybrid approach is more efficient and effective than a purely visual technique, we still were able to create an aggregated list of usability concerns based on both evaluations for the formula editor. These issues have been the basis for our recommendations, which include a revalidation of the terminology used, the inclusion of an expert mode as well as a redesign of tap target sizes and the addition of visual clues within the interface.