What it measures: the Alternate Floating Algae Index (AFAI), a satellite method (Hu et al., 2015) that detects sargassum's distinct light signature against open water, developed by the University of South Florida. It's shown here as a 7-day cumulative composite rather than a single day's pass, since daily satellite coverage has real gaps from cloud cover and sun glint that the cumulative version smooths over.

Resolution: the underlying satellite data (MODIS/VIIRS) resolves to about 1.5-2 km per pixel -- at Anguilla's scale, that's genuinely coarse, which is why the layer looks blocky up close. This is a real limit of the current public satellite product, not a display setting -- USF/NOAA are actively developing higher-resolution regional products (currently piloted for Puerto Rico/USVI) that combine finer imagery with ocean circulation models to forecast movement days in advance; that capability isn't yet available for Anguilla specifically.

Which way it's heading: there's no precise per-location trajectory forecast publicly available for this area yet. In general, sargassum in this part of the Caribbean is carried westward by the Caribbean Current, with local wind and eddies causing real day-to-day variation -- so "generally moving west, unpredictably" is the honest general answer, not a specific heading.

Where the actual forecast comes from: USF's Optical Oceanography Lab publishes a real monthly bulletin with expert regional outlooks (linked above) -- that's the closest thing to a genuine "forecast" that currently exists for this phenomenon; a true 10-day forecast doesn't yet exist publicly for this specific area.