The successful use of remote sensing for mapping marine macrophytes is based on maximizing their spectral contrast within their surroundings. To maximize the contrast, remote sensing mapping techniques must be calibrated with accurate and reproducible reflectance spectra of the macrophyte. However, when the macrophyte occurs as thin leaf (thalli) layered units, the ability to obtain accurate and reproducible reflectance spectra is complicated. Not correcting for sequential additions from layered recordings could hinder sample-to-sample comparability, and thereby the remote sensing mapping performance, particularly with respect to assessment of macrophyte condition. In this study, the accuracy of flat-plate spectral measurements (400–1,000 nm) of the thin-leaf (thalli) marine macrophyte Ulva curvata was determined. A laboratory experiment measured the progressively increasing reflectance with the sequential addition of Ulva thalli in the visible (VIS) and near-infrared (NIR). Confirmation of the experimental results was obtained by simulating the same progressive reflectance increase with a mathematical calculation based on an Ulva thallus diffuse reflectance weighted by a transmittance power series. These results confirmed that common field spectral measurements of layered thalli (or leaves) can produce incorrect estimates of the intrinsic reflectance of macro algae. Methods for correction of the flat-plate spectral measurements of layered thalli were provided.