Your plant may have spider mites. Check on the underside of the leaves for what looks like small grains of salt. If you see them then apply soap and water at a ratio of 5ml to one litre three times at five to seven day intervals.
The light and dark parts on the leaves indicate that this plant has a nutrient deficiency. As this site indicates:
Guava trees are susceptible to iron deficiency under alkaline and high
pH soil conditions (e.g., rockland soils, calcareous sands)
You should apply iron sulfate or a chelated micronutrient mix up to twice a year according to the instructions on the fertilizer label.
Repotting with a soil mix that has some compost or other organic matter should help too. The plant is potted in a peat based mix that works well for growers but does not have the same ability to hold micro nutrients over time as soil does due to the lack of clay.