While there are various types of filamentous algae types, they all have similar base causes & triggers. The common trigger is a spike in organic waste/ammonia. This may occur due to heavy usage of ammonia/urea rich fertilizer. (such as burying osmocote not deep enough). Dead livestock, poorly functioning filters, damaged or stressed plants can all be sources of ammonia. High light is an accelerator but by itself alone seldom trigger filamentous algae.
If found growing on or attached to even fast growing stem plants - this generally means that plants are either damaged, stressed or deficient.
When algae preferentially attaches to plants rather than hardscape, it happens because plants have deteriorating leaves ( due to age/nutrient deficiencies/damage ) or are under stress (transplant stress/pruning/unstable parameters etc). Plants under harsh/fluctuating conditions continually re-program their enzymes to try to adapt to new conditions - older DNA is ejected from the leaves as organic waste & ammonia. This becomes a chemical signal for algae to attach opportunistically; the deteriorating leaves feed algae by releasing carbohydrates and sugars; if the plant is not dosing well it means that the algae will not be shaded over by new growth. This is how simple chemical triggers can allow algae to compete against higher level plants. In such situations, stabilizing the tank and improving plant health is critical.
Healthy, well maintained planted tanks should be completely free of filamentous algae.
Plants can show signs of growth even under stress - an unstable tank can still show regular plant growth. Signs of stress include; algae on leaves, premature shedding of older leaves (bare lower stems), uneven leaf sizes/shapes/coloration, stunted growth tips, elongated inter-nodes, thin/brittle stems & leaves.
When left unattended, deteriorating leaves combined with high light levels can result in algae ( thread / hair algae above, for instance) taking over a tank completely.
Watch video on how to change water for planted tank.